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Research · Supplements

Fish oil

A Cochrane review of 86 trials found long-chain omega-3 lowered triglycerides by about 15% and made little or no difference to death or cardiovascular events overall. Coronary heart disease events fell slightly. Pooled outcome trials found more atrial fibrillation, especially above 1 g a day. Bleeding did not rise overall.

Add to Bio HackChecked 2026-11-13
Status
Dietary supplement in the US. Prescription omega-3 products (omega-3-acid ethyl esters, icosapent ethyl) are FDA-approved drugs.
Also called
omega-3, omega-3 fatty acids, n-3 PUFA, EPA, DHA, eicosapentaenoic acid, docosahexaenoic acid, krill oil, algal oil, cod liver oil, icosapent ethyl, Lovaza, Vascepa
Taken for
heart health, triglycerides, cholesterol, inflammation, depression, mood, brain health, joint pain, muscle mass, recovery, rheumatoid arthritis, Crohn's disease, ulcerative colitis, inflammatory bowel disease, psoriasis, lupus, autoimmune disease, gum disease, periodontitis, skin aging, skin hydration, skin elasticity, period pain

Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on

Shown to help10

Reported risks6

What people take it for, and what was found

Shown to help10

Strong

Triglycerides

Shown to lower triglycerides by about 15% in adults, more at higher doses.

The numbers and sources
Effect
Increasing long-chain omega-3 reduced triglycerides by about 15% in a dose-dependent way (high-certainty evidence). It had little or no effect on other blood lipids, adiposity or blood pressure [1]. In 23 RCTs of people with coronary heart disease, omega-3 lowered triglycerides (SMD -0.25, 95% CI -0.38 to -0.11), with a linear dose-response [2]. Across 21 trials in mixed populations, triglycerides fell 16.95 mg/dL (95% CI -23.25 to -10.66) [3]. In 9 trials of people with fatty liver disease, they fell 13.81 mg/dL (95% CI -24.56 to -3.06), with no significant effect in those who also had type 2 diabetes [4]. In 21 trials of metabolic syndrome, reductions ranged from about 25 to 57 mg/dL, largest above 2,000 mg a day [5].
Population
86 RCTs with 162,796 adults at varying cardiovascular risk, lasting 12 to 88 months, mostly capsule supplements [1]; 2,061 patients with coronary heart disease [2]; 2,043 adults in 24 studies [3]; 528 patients with fatty liver disease [4]; about 1,950 adults with metabolic syndrome or its components [5].
Sources
[1], [2], [3], [4], [5]
Caveats
The Cochrane abstract reports the triglyceride effect as a percentage without a confidence interval [1]. The metabolic syndrome review reports subgroup reductions about three times the size of the other pooled estimates, from dose and duration subgroups rather than one overall estimate [5].
Limited

Heart attack, stroke and cardiac death in people on hemodialysis

One trial found about 43% fewer serious cardiovascular events with 4 g a day in adults on hemodialysis, who have very high cardiovascular risk.

The numbers and sources
Effect
Over 3.5 years, serious cardiovascular events occurred at 0.31 per 1,000 patient-days on fish oil and 0.61 on a corn-oil placebo (HR 0.57, 95% CI 0.47 to 0.70). Cardiac death (HR 0.55), heart attack (HR 0.56), stroke (HR 0.37) and amputation for vascular disease (HR 0.57) were each lower. Adherence and adverse events did not differ meaningfully between groups [7].
Population
1,228 adults on maintenance hemodialysis at 26 sites in Canada and Australia, given 4 g of omega-3 fatty acids a day (1.6 g EPA and 0.8 g DHA) [7].
Sources
[7]
Caveats
One trial, in people on hemodialysis, who have a very high cardiovascular risk. It does not describe the general population [7].
Why limited
This rests on one trial, of 1,228 people. Strength counts independent studies that agree, so one trial is limited at any size.
Strong

Periodontitis, added to dental treatment

Shown to reduce gum pocket depth and improve attachment by about 0.5 mm when added to scaling and root planing in adults with periodontitis.

The numbers and sources
Effect
Across 15 RCTs (617 patients), omega-3 added to scaling and root planing reduced probing pocket depth by 0.43 mm more (95% CI -0.68 to -0.18, I² 92%) and improved clinical attachment level by 0.47 mm more (95% CI -0.76 to -0.18, 12 RCTs) than scaling alone or with placebo. Gingival index (SMD -0.59), bleeding on probing (SMD -0.95) and plaque index (SMD -0.34) also improved [20]. A review of 21 studies found attachment gain 0.54 mm greater (95% CI 0.36 to 0.72) and pocket depth 0.57 mm lower (95% CI 0.4 to 0.74), with larger effects at 6 to 12 months [21]. A third review of 16 RCTs, some adding aspirin, found attachment 0.49 mm better (95% CI -0.75 to -0.23) and pocket depth 0.44 mm lower at 3 months, and 0.58 mm and 0.45 mm at 6 months [22].
Population
Adults with periodontitis receiving nonsurgical periodontal therapy [20][21][22].
Sources
[20], [21], [22]
Caveats
Certainty of evidence was very low to low [20] and moderate to low [21], and heterogeneity was very high in both. The reviews overlap in the trials they include. The differences are fractions of a millimeter. This is omega-3 alongside scaling and root planing, not instead of it.
Moderate

Depression symptoms

Shown to lower depression symptoms modestly in people with symptoms, with EPA-rich formulas only. Not shown to prevent depression.

The numbers and sources
Effect
In 26 double-blind trials (2,160 participants) of people with depression symptoms, omega-3 lowered symptom scores by an SMD of -0.28. Formulations with 60% or more EPA at 1 g a day or less showed benefit (SMD -1.03 for EPA-major), and DHA-pure or DHA-major formulations did not [23]. For prevention, 31 trials (41,470 participants) found little or no effect on the risk of depression symptoms (RR 1.01, 95% CI 0.92 to 1.10) or anxiety symptoms [24].
Population
People with depression symptoms in treatment trials [23]; adults with or without depression followed for 6 to 12 months or more in prevention trials [24].
Sources
[23], [24]
Caveats
The treatment and prevention reviews ask different questions. In [24], effects on depression severity in existing depression were rated very-low-quality evidence.
Moderate

Lupus disease activity

Shown to lower lupus disease activity about 0.9 SLEDAI points at 3 g a day or less in adults with lupus, at low certainty.

The numbers and sources
Effect
Across 5 RCTs (274 patients), omega-3 at 3 g a day or less reduced lupus disease activity (SMD -0.33, 95% CI -0.57 to -0.09), about 0.9 SLEDAI points (95% CI -1.6 to -0.3) [25]. A systematic review of 13 trials found 8 of 10 that measured disease activity reported improvement. Kidney measures, C3, C4 and anti-dsDNA did not change [26].
Population
Adults with systemic lupus erythematosus, 80% or more women, over 12 to 52 weeks [25][26].
Sources
[25], [26]
Caveats
The meta-analysis rated certainty as low [25]. The systematic review describes the evidence as conflicting [26].
Moderate

Muscle soreness and recovery after exercise

Shown to reduce muscle soreness and improve strength after eccentric exercise in healthy adults, though the wider review called recovery effects equivocal.

The numbers and sources
Effect
In 9 placebo-controlled eccentric exercise trials, long-chain omega-3 reduced delayed onset muscle soreness (Hedges' g -0.75, 95% CI -1.14 to -0.36), creatine kinase (g -0.40) and swelling (g -0.45), and improved strength (g 0.45, 95% CI 0.07 to 0.83) and range of motion (g 0.93) at peak impairment. Across all 43 studies reviewed, the effect on recovery was equivocal [30].
Population
Healthy young to middle-aged adults [30].
Sources
[30]
Caveats
The authors note significant methodological limits and could not assess dosing. One author reports product and placebo support from AkerBiomarine, a krill oil producer [30].
Limited

Muscle mass and function in older adults

Limited evidence: omega-3 added about 0.33 kg of muscle mass and slightly improved timed up and go in older adults.

The numbers and sources
Effect
Across 10 trials in older adults, omega-3 added 0.33 kg of muscle mass (95% CI 0.05 to 0.62) and shortened timed up and go by 0.30 seconds (95% CI -0.43 to -0.17). Above 2 g a day the muscle mass gain was 0.67 kg (95% CI 0.16 to 1.18) [34]. In 94 healthy older adults, 4 g a day of krill oil for 6 months increased muscle strength and size against a control oil, with similar responses by age, sex and BMI [35].
Population
Older adults in 10 RCTs [34]; adults 65 and older doing less than 1 hour of structured exercise a week [35].
Sources
[34], [35]
Caveats
Few trials, and the dose subgroup is post hoc. The search ended in July 2018 [34]. The krill oil report is a secondary analysis of one trial [35].
Limited

Skin hydration, water loss and elasticity

Small trials found krill oil and fish oil raised skin hydration and lowered water loss. The krill trials were funded by the oil supplier.

The numbers and sources
Effect
In two placebo-controlled pilot trials, krill oil at 1 g a day (51 adults) or 2 g a day (50 adults) for 12 weeks lowered transepidermal water loss (from 14.47 to 13.83 and from 14.25 to 13.02 g/m²/h) and raised skin hydration and elasticity against placebo. The changes tracked the rise in omega-3 index [38]. In 60 people on hemodialysis with severe itch, 1,000 mg of fish oil (more than 900 mg EPA) twice a day for 3 months raised skin moisture on the face and arms, which did not rise on a soybean oil placebo, and itch scores fell [39]. A meta-analysis of 61 human diet and supplement studies found that lipids and fatty acids as a group improved skin hydration (pSMD 0.54, 95% CI 0.28 to 0.80) and elasticity (pSMD 0.49, 95% CI 0.14 to 0.83) and reduced wrinkles (pSMD -0.62, 95% CI -0.92 to -0.31) [40].
Population
Healthy adults with baseline water loss of more than 10 and up to 24.9 g/m²/h [38]; adults on maintenance hemodialysis [39]; participants in 61 studies [40].
Sources
[38], [39], [40]
Caveats
Two krill trial authors are employees of Aker BioMarine, which provided the krill and placebo oils and funded the study [38]. The meta-analysis grouped all lipids and fatty acids, so it does not isolate fish oil [40]. The hemodialysis abstract reports within-group changes [39].
Limited

Sunburn and the skin's response to UV light

Small trials found purified EPA or fish oil raised the UV dose needed to cause sunburn. Skin aging and skin cancer were not studied.

The numbers and sources
Effect
In 42 healthy adults, 4 g a day of purified EPA for 3 months raised the UV dose needed to cause sunburn from a mean 36 to 49 mJ/cm² (P < 0.01) and lowered UV-induced p53-positive skin cells from 16 to 8 per 100 epidermal cells. No change appeared on the oleic acid comparison [41]. In 20 people, 4 weeks of fish oil raised the minimal erythema dose slightly but significantly, with no change on placebo [42]. In 79 women with nickel allergy, 5 g a day of an EPA-rich lipid for 3 months reduced UV suppression of skin immunity by 6.9% against a control lipid (95% CI -2.1% to 15.9%), which was not significant. At the middle UV dose the difference was 11% (95% CI 0.5% to 21.4%) [43]. A systematic review of 47 photoprotection studies included omega-3 trials and placed the most evidence with polyphenols, carotenoids and Polypodium leucotomos [44].
Population
42 healthy adults [41]; 20 adults [42]; 79 women aged 22 to 60 with fair skin (phototype I or II) [43]; human studies [44].
Sources
[41], [42], [43], [44]
Caveats
Small, short trials of acute UV markers, not of skin aging or skin cancer. The EPA trial reported within-group changes [41].
StrongStudied in women

Period pain

Shown in two reviews to lower period pain more than controls in women with dysmenorrhea, at 300 to 1,800 mg a day for 2 to 3 months.

Compared with control groups the source abstracts do not describe [54][55]

The numbers and sources
Effect
Across 12 studies (881 women) giving 300 to 1,800 mg of long-chain omega-3 a day for 2 or 3 months, 8 pooled studies showed lower pain (Cohen's d -1.020, 95% CI -1.53 to -0.51). 86% of studies that measured pain medicine use found it fell, and few mild side effects were reported [54]. A second review of RCTs found lower primary dysmenorrhea severity (SMD -1.075, 95% CI -1.871 to -0.279), with smaller effects at higher daily intakes and in older women [55].
Population
881 women with dysmenorrhea in 12 studies [54]; women with primary dysmenorrhea in RCTs [55].
Sources
[54], [55]
Caveats
Most studies were rated neutral in quality [54]. The trials were small and short, and the two reviews share many of them.

Mixed results6

Moderate

LDL and HDL cholesterol

Mixed results: most reviews found little or no change in LDL or HDL, while subgroups saw LDL fall in some groups and rise in others.

The numbers and sources
Effect
The Cochrane review found little or no effect on blood lipids other than triglycerides [1]. In coronary heart disease, LDL and HDL did not change significantly, and total cholesterol fell slightly (SMD -0.12) [2]. Across mixed populations, HDL rose 1.55 mg/dL (95% CI 0.69 to 2.42), and LDL fell only in subgroups of women and of people with polycystic ovary syndrome [3]. In metabolic syndrome trials, LDL rose in low-dose subgroups (7.04 mg/dL short term, 35.5 mg/dL long term) [5].
Population
Trial participants in the four reviews above [1][2][3][5].
Sources
[1], [2], [3], [5]
Caveats
The reviews differ on direction in subgroups. The LDL rise in [5] came from few studies per subgroup.
Moderate

Cardiovascular outcomes with purified EPA (prescription)

Mixed results: one review found fewer cardiovascular deaths with prescription purified EPA, and another found no change in cardiovascular death at high doses.

The numbers and sources
Effect
Across 16 RCTs (127,771 patients, median 3.7 years), cardiovascular death fell with purified EPA (HR 0.79, 95% CI 0.67 to 0.94) and less with mixed EPA and DHA (HR 0.92, 95% CI 0.84 to 1.00) [8]. Across 6 trials (42,738 adults) of 1.8 g a day or more of EPA-dominant omega-3, hospitalization for unstable angina fell (RR 0.75, 95% CI 0.66 to 0.87), while cardiovascular death, ischemic stroke and hs-CRP did not change significantly. Recurrent heart attack and revascularization fell only after excluding STRENGTH, the one mixed EPA and DHA outcome trial [9]. Across 10 imaging trials (860 patients) on statins, total coronary plaque volume changed by -10.0% with added EPA, -3.3% with added EPA and DHA and +1.9% on statin alone [10]. In 23 RCTs of omega-3 in general, plaque volume did not change [2].
Population
Adults in primary or secondary prevention trials [8]; adults with established cardiovascular disease or high risk [9]; patients with coronary artery disease [10][2].
Sources
[8], [9], [10], [2]
Caveats
Purified EPA (icosapent ethyl) is a prescription drug, not an over-the-counter fish oil. The two outcome reviews disagree on cardiovascular death [8][9]. Authors of [8] and [10] include a REDUCE-IT investigator with Amarin research funding and an Amarin speakers bureau member [8].
Moderate

Rheumatoid arthritis

Mixed results: one review found better disease activity markers in rheumatoid arthritis, while others found small or no effects on pain and swollen joints.

The numbers and sources
Effect
Across 20 RCTs (1,252 patients), omega-3 improved eight disease activity markers. Of five inflammatory markers, only leukotriene B4 fell (SMD -0.440, 95% CI -0.676 to -0.205) [11]. Across 23 placebo-controlled trials, pooled effects on pain (SMD -0.16, 95% CI -0.40 to 0.09), tender joints (SMD -0.20, 95% CI -0.46 to 0.05) and swollen joints (SMD -0.10, 95% CI -0.28 to 0.07) were small, with intervals that include no effect [12]. Across 18 RCTs (1,018 patients), tender joint count fell (SMD -0.59, 95% CI -0.79 to -0.39), while ESR, CRP and DAS28 did not change significantly [13].
Population
Adults with rheumatoid arthritis in RCTs [11][12][13].
Sources
[11], [12], [13]
Caveats
The certainty of the evidence was mostly very low or low [12]. The authors of [12] note that earlier findings of lower NSAID use may reflect inadequate blinding.
Moderate

Chronic pain and knee osteoarthritis

Mixed results: omega-3 lowered chronic pain but not osteoarthritis pain in one review, while krill oil reviews found better knee osteoarthritis pain scores.

The numbers and sources
Effect
Across 41 RCTs (3,759 participants), omega-3 lowered chronic pain intensity (SMD -0.55, 95% CI -0.76 to -0.34, I² 87%), from -0.27 at 1 month to -0.83 at 6 months. Effects were significant in rheumatoid arthritis, migraine and mixed chronic pain, and not in osteoarthritis or breast pain [27]. In knee osteoarthritis, krill oil improved WOMAC pain (SMD -0.60, 95% CI -0.99 to -0.21), stiffness and function across 5 RCTs (730 participants), with no change on a visual analog pain scale [28]. A 2026 review of 6 RCTs (971 participants) also found better WOMAC pain and function at week 4 [29].
Population
Adults with chronic pain conditions [27]; adults with knee osteoarthritis [28][29].
Sources
[27], [28], [29]
Caveats
Heterogeneity was high in [27]. The reviews disagree on osteoarthritis, though the krill reviews tested one product type. Lower doses (1.35 g a day or less) did as well as higher doses [27].
Limited

Inflammation markers (CRP)

Limited evidence: CRP fell in hemodialysis patients but not with high-dose EPA, in knee osteoarthritis, or in most trials of trained people.

The numbers and sources
Effect
In 13 hemodialysis trials (678 patients), omega-3 in triglyceride form lowered CRP (SMD -0.62, 95% CI -1.22 to -0.03, I² 74%), and so did doses under 2,000 mg a day (SMD -0.32) [32]. In 3 trials of trained people, only 1 found a significant drop in CRP after exercise [33]. High-dose EPA did not change hs-CRP across 6 outcome trials [9], and krill oil did not change CRP in knee osteoarthritis [29].
Population
Hemodialysis patients [32]; trained individuals [33]; cardiovascular and osteoarthritis trials [9][29].
Sources
[32], [33], [9], [29]
Caveats
The reviews disagree by population. In rheumatoid arthritis, only leukotriene B4 of five inflammatory markers fell [11].
Limited

Preventing autoimmune disease

One trial found autoimmune disease was not significantly lower on 1,000 mg a day over 5.3 years. Two more years of follow-up found 17% fewer cases.

The numbers and sources
Effect
In VITAL, 1,000 mg a day of marine omega-3 for a median 5.3 years led to confirmed autoimmune disease in 130 people against 148 on placebo (HR 0.85, 95% CI 0.67 to 1.08, P 0.19) [36]. With 2 more years of follow-up after the trial, the HR was 0.83 (95% CI 0.70 to 0.99), with 234 cases against 280 [37].
Population
25,871 US men 50 and older and women 55 and older [36]. 21,592 followed for 2 more years [37].
Sources
[36], [37]
Caveats
Both sources are the same trial. The randomized result did not reach significance, and the later result includes cases after the trial ended [36][37].
Why limited
This rests on one trial, of 25,871 people. Strength counts independent studies that agree, so one trial is limited at any size.

Not shown to help4

Strong

Death and cardiovascular events

Not shown to change deaths, cardiovascular events or stroke in adults. Coronary events and heart attacks fell slightly, by about 8% to 9%.

The numbers and sources
Effect
Long-chain omega-3 made little or no difference to all-cause mortality (RR 0.97, 95% CI 0.93 to 1.01, 45 RCTs), cardiovascular events (RR 0.96, 95% CI 0.92 to 1.01, 43 RCTs) or stroke (RR 1.02, 95% CI 0.94 to 1.12). Coronary heart disease events fell slightly (RR 0.91, 95% CI 0.85 to 0.97, number needed to treat 167, low certainty) [1]. A meta-analysis of 13 large trials (127,477 people, mean 5.0 years) found lower myocardial infarction (RR 0.92, 95% CI 0.86 to 0.99), coronary death (RR 0.92, 95% CI 0.86 to 0.98) and total cardiovascular disease (RR 0.97, 95% CI 0.94 to 0.99) with REDUCE-IT excluded, and a linear dose-response for total cardiovascular disease [6].
Population
143,693 adults for mortality [1]; 127,477 adults in 13 cardiovascular outcome trials [6].
Sources
[1], [6]
Caveats
The two reviews agree on small effect sizes and differ in emphasis. Adding REDUCE-IT, a trial of high-dose purified EPA, strengthened the associations but introduced significant heterogeneity [6]. Effects in [1] did not differ by dose or trial duration.
Strong

Crohn's disease and ulcerative colitis

Not shown to prevent relapse of Crohn's disease in the two large low-bias trials, or of ulcerative colitis.

The numbers and sources
Effect
A Cochrane review of 6 studies (1,039 patients) found 39% relapsed on omega-3 at 12 months against 47% on placebo (RR 0.77, 95% CI 0.61 to 0.98). In the two large low risk-of-bias trials (738 patients) it was 37% against 42% (RR 0.88, 95% CI 0.74 to 1.05) [14]. In those two trials, 4 g a day for up to 58 weeks did not change relapse at 1 year (31.6% against 35.7%, HR 0.82, and 47.8% against 48.8%, HR 0.90) [15]. In ulcerative colitis, 3 studies (138 patients) found relapse rates similar to placebo (RR 1.02, 95% CI 0.51 to 2.03) [16]. Across 13 RCTs in people with bowel disease, relapse had RR 0.85 (95% CI 0.72 to 1.01), from low-quality evidence [17].
Population
Patients with Crohn's disease in remission [14][15]. People with ulcerative colitis in remission [16]. Adults in trials of 24 weeks or more [17].
Sources
[14], [15], [16], [17]
Caveats
The Cochrane authors rated the pooled Crohn's result very low quality because of heterogeneity and publication bias [14]. The ulcerative colitis review had few patients [16].
Strong

Psoriasis severity

Not shown to reduce psoriasis severity on its own in adults. Added to conventional treatment, PASI scores fell about 4 points.

The numbers and sources
Effect
Across 3 RCTs (337 participants) with usable data, fish oil did not reduce PASI (MD -0.28, 95% CI -1.74 to 1.19) [18]. A review of 18 RCTs (927 participants) found fish oil alone had no effect on PASI (P 0.47), lesion area or itch. Combined with conventional treatment, PASI fell by 3.92 points (95% CI -6.15 to -1.69) [19].
Population
Adults with psoriasis [18][19].
Sources
[18], [19]
Caveats
Only 3 of 13 trials had usable data in [18].
Moderate

Cognition in Alzheimer disease

Not shown to change cognition scores in people with Alzheimer disease across 4 trials.

The numbers and sources
Effect
Across 4 RCTs (702 patients with Alzheimer disease), ADAS-Cog scores did not differ significantly from placebo (MD 1.37, 95% CI 0.00 to 2.73, p 0.05, I² 35%) [31].
Population
376 patients on omega-3 and 326 on placebo [31].
Sources
[31]
Caveats
Few trials. Higher ADAS-Cog scores mean worse cognition, and the interval reaches no difference [31].

Claimed, not studied in people1

Untested

Fibroids

Not studied in people. In the lab, EPA and DHA changed the membranes of human fibroid cells and lowered genes tied to mechanical signaling and lipid handling.

No study in people. This rests on lab studies.

The numbers and sources
Effect
In primary human fibroid and normal uterine muscle cells, EPA or DHA raised polyunsaturated fat content, lowered membrane fluidity, and reduced expression of ABCG1, ABCA1 and AKAP13. Genes for extracellular matrix proteins such as collagen and fibronectin did not change [50].
Population
Primary human fibroid and uterine muscle cells [50].
Sources
[50]
Caveats
Cell studies only. No trial of fish oil in women with fibroids or heavy bleeding was located, and cohorts of omega-3 from food give mixed results (see Risks).

Doses studied

The Cochrane trials gave long-chain omega-3 from 0.5 g a day to more than 5 g a day, and 19 trials gave at least 3 g a day [1]. Prevention trials of depression used a median 0.95 g a day for 12 months [24]. Atrial fibrillation trials were split into 1 g a day or less and more than 1 g a day [45]. The muscle trials that showed larger gains used more than 2 g a day [34], and the krill oil muscle trial used 4 g a day [35]. Triglyceride reductions in metabolic syndrome were largest above 2,000 mg a day [5]. Purified EPA outcome trials used 1.8 g a day or more [9]. The Crohn's disease trials gave 4 g a day of omega-3 free fatty acids [15], and VITAL gave 1,000 mg a day [36]. The skin trials gave 1 or 2 g of krill oil a day for 12 weeks [38], 4 g of purified EPA a day for 3 months [41] and 5 g a day of a lipid with 70% EPA and 10% DHA for 3 months [43].

Risks

Reported risks4

ModerateHow often is not known

Atrial fibrillation

Shown to raise atrial fibrillation about 25% in large cardiovascular trials, more above 1 g a day. A Cochrane review found no rise in arrhythmia overall.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
In 7 cardiovascular outcome trials (81,210 patients, mean age 65, weighted follow-up 4.9 years), marine omega-3 was associated with more atrial fibrillation (HR 1.25, 95% CI 1.07 to 1.46). The hazard ratio was 1.49 (95% CI 1.04 to 2.15) in trials above 1 g a day and 1.12 (95% CI 1.03 to 1.22) at 1 g a day or less [45].
Population
Adults in large cardiovascular trials, 39% female [45].
Sources
[45]
Caveats
The Cochrane review found no difference in arrhythmia overall (RR 0.99, 95% CI 0.92 to 1.06, low certainty) [1]. The two analyses used different trial sets and outcome definitions.
ModerateHow often is not known

Digestive side effects

Shown to raise diarrhea and upper digestive symptoms in Crohn's disease trials, with no serious adverse events recorded.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
In Crohn's disease trials, diarrhea (RR 1.36, 95% CI 1.01 to 1.84, 4 studies, 862 patients) and upper digestive symptoms (RR 1.65, 95% CI 1.25 to 2.18, 5 studies, 999 patients) were more common on omega-3. No serious adverse events were recorded [14]. Of 13 hemodialysis trials, 8 reported no adverse events and 5 described only mild, transient effects such as diarrhea [32].
Population
Patients with Crohn's disease in maintenance trials [14]; hemodialysis patients [32].
Sources
[14], [32]
Caveats
Adverse events were poorly reported in the hemodialysis trials [32].
LimitedHow often is not known

Interaction with warfarin

Case reports: in a woman on warfarin, INR rose from 2.8 to 4.3 after her fish oil dose doubled, and fell after it was reduced.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
In a 67-year-old woman on stable warfarin, doubling fish oil from 1,000 to 2,000 mg a day was followed by INR rising from 2.8 to 4.3 within a month. INR fell to 1.6 one week after the fish oil was reduced [47].
Population
One case report [47].
Sources
[47]
Caveats
A single case. It shows a temporal association, not a measured rate.
LimitedHow often is not known

Label content and oxidation

Limited evidence: product tests found most fish oils held less EPA or DHA than labeled, and most in New Zealand exceeded oxidation limits.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Of 32 fish oil supplements sold in New Zealand, 3 met their labeled EPA and DHA content, and 69% contained less than 67% of the label. 83% exceeded peroxide value limits and 8% met all international oxidation thresholds [48]. In 47 US fish, krill and algal oil products, EPA ranged from 66% to 184% of label, and over 70% did not contain the stated amount of EPA or DHA [49].
Population
Product testing [48][49].
Sources
[48], [49]
Caveats
Two markets, tested more than a decade ago. Price, expiry date and country of origin did not track quality in [48].

Mixed results2

ModerateHow often is not known

Bleeding

Bleeding was about as common as on control across 11 trials. High-dose purified EPA raised bleeding risk by an absolute 0.6%.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Across 11 RCTs (120,643 patients), bleeding did not differ from control (RR 1.09, 95% CI 0.91 to 1.31), including hemorrhagic stroke, intracranial and gastrointestinal bleeding. High-dose purified EPA raised relative bleeding risk by 50%, an absolute increase of 0.6%. Bleeding was associated with EPA dose (risk difference 0.24, 95% CI 0.05 to 0.43) and not with background antiplatelet therapy [46].
Population
Patients in cardiovascular RCTs [46].
Sources
[46]
Caveats
Trials enrolled patients able to join outcome trials. Warfarin users are reported separately below.
LimitedStudied in womenHow often is not known

Fibroids with higher marine omega-3 intake from food

Mixed results: two cohorts of Black women linked the highest marine omega-3 or DHA intake from food to 18% to 49% more fibroids. A larger cohort found no link.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
In 12,044 premenopausal Black women followed 8 years, the highest fifth of marine omega-3 intake was linked to more fibroid diagnoses than the lowest (IRR 1.18, 95% CI 1.05 to 1.34), and dark-meat fish to IRR 1.13 (95% CI 1.00 to 1.28) [51]. In 1,171 Black women screened by ultrasound over 5 years, the highest quarter of DHA intake was linked to 49% more new fibroids (HR 1.49, 95% CI 1.04 to 2.14), and total marine omega-3 to HR 1.35 (95% CI 0.94 to 1.93) [52]. In 81,590 US nurses followed 18 years, no type of dietary fat was linked to fibroids. In 553 of them with blood tests, the highest third of red blood cell omega-3 was linked to lower odds of fibroids (OR 0.41, 95% CI 0.19 to 0.89) [53].
Population
Premenopausal Black women in the US [51][52]; premenopausal US nurses aged 25 to 42 at enrollment [53].
Sources
[51], [52], [53]
Caveats
Observational, and these studies measured omega-3 from food, not fish oil supplements. The authors of [52] note that pollutants in fish, rather than the fatty acids, may drive the link. Diet tracks other habits, so confounding is possible.

Claims tested and not found

  1. That fish oil lowers the risk of death. All-cause mortality did not differ from control across 45 RCTs (RR 0.97, 95% CI 0.93 to 1.01) [1].
  2. That fish oil prevents depression. Prevention trials found no change in the risk of depression symptoms (RR 1.01, 95% CI 0.92 to 1.10) [24].
  3. That fish oil keeps Crohn's disease in remission. In two large low risk-of-bias trials, relapse did not differ from placebo (RR 0.88, 95% CI 0.74 to 1.05) [14][15].
  4. That fish oil alone improves psoriasis. PASI did not change with fish oil alone (P 0.47) [19] or in pooled trials (MD -0.28) [18].
  5. That fish oil improves cognition in Alzheimer disease. ADAS-Cog scores did not differ significantly from placebo across 4 RCTs [31].
  6. That omega-3 prevents UV suppression of skin immunity. Across three UV doses, the difference from a control lipid was not significant (6.9%, 95% CI -2.1% to 15.9%) [43].

Sources

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  2. Zhang C, Xie Y, Zhou J, et al. Effects of Omega-3 Fatty Acids Intake on Lipid Metabolism and Plaque Volume in Patients With Coronary Heart Disease: A Systematic Review and Dose-Response Meta-Analysis of Randomized Clinical Trials. Food Science & Nutrition. 2025. https://doi.org/10.1002/fsn3.70372
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  4. Li L, Cong C, Zhang X, Zhang X. The Impact of Omega-3 Fatty Acids on Triglyceride Levels in Patients with Metabolic Dysfunction-Associated Steatotic Liver Disease: A Meta-Analysis of Randomized Controlled Trials with Subgroup Analysis by Diabetes Status. Diabetes, Metabolic Syndrome and Obesity. 2026. https://doi.org/10.2147/DMSO.S573711
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  6. Hu Y, Hu FB, Manson JE. Marine Omega-3 Supplementation and Cardiovascular Disease: An Updated Meta-Analysis of 13 Randomized Controlled Trials Involving 127 477 Participants. Journal of the American Heart Association. 2019. https://doi.org/10.1161/JAHA.119.013543
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