What does the research show about fat burners?

Fat burners

A meta-analysis of thermogenic supplements found weight and fat changes with confidence intervals crossing zero. Ephedra and ephedrine added about 0.9 kg a month of loss in short trials, with 2.2 to 3.6 times the odds of psychiatric, autonomic and heart symptoms. FDA declared ephedra supplements adulterated in 2004. FDA found sibutramine in 269 of 317 adulterated weight loss products.

Add to Bio HackChecked 2026-11-09

Status: Sold as dietary supplements in the US. FDA declared dietary supplements containing ephedrine alkaloids (ephedra) adulterated in a rule that took effect April 12, 2004. Synephrine and yohimbe are sold as supplement ingredients.
Also called: thermogenics, thermogenic supplements, weight loss supplements, synephrine, p-synephrine, bitter orange, Citrus aurantium, Advantra Z, yohimbine, yohimbe, Pausinystalia johimbe, ephedra, ma huang, ephedrine, Hydroxycut, stim fat burner
Taken for: fat loss, weight loss, metabolism, appetite, energy, cutting, pre-workout

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 help3

ModerateShown to add about 0.6 to 1.0 kg a month of weight loss over placebo in adults, with no trial longer than 6 months.
LimitedLimited evidence: a review reported modest weight loss with bitter orange products, mostly in combinations and often with caffeine. The authors consulted for a bitter orange marketer.
LimitedOne small trial found body fat fell from 9.3% to 7.1% over 21 days in male soccer players, with no change in performance.

Reported risks4

StrongShown to raise psychiatric, autonomic and gut symptoms and palpitations 2.2 to 3.6 times. FDA declared ephedrine alkaloid supplements adulterated in 2004.
ModerateFDA identified 317 adulterated weight loss supplements from 2007 to 2016, 84.9% containing sibutramine, a drug withdrawn from the market.
LimitedCase reports: poison control yohimbine cases rose from 2000 to 2006, with gut distress, fast heart rate and anxiety. Most labeled amounts were inaccurate.
LimitedSmall trials found no heart rate or blood pressure rise with bitter orange alone. Case reports describe chest pain, arrhythmias and stroke after multi-ingredient products.

What people take it for, and what was found

Shown to help3

Moderate

Weight loss with ephedra and ephedrine

Shown to add about 0.6 to 1.0 kg a month of weight loss over placebo in adults, with no trial longer than 6 months.

The numbers and sources

Effect: Compared with placebo, weight loss was 0.6 kg a month more with ephedrine (5 trials), 1.0 kg with ephedrine plus caffeine (12 trials), 0.8 kg with ephedra (1 trial) and 1.0 kg with ephedra plus caffeine-containing herbs (4 trials). No trial lasted longer than 6 months [2].
Population: Adults in controlled weight loss trials of at least 8 weeks [2].
Sources: [2]
Caveats: No long-term data. Ephedra supplements are no longer legal in the US [3].

Limited

Synephrine (bitter orange)

Limited evidence: a review reported modest weight loss with bitter orange products, mostly in combinations and often with caffeine. The authors consulted for a bitter orange marketer.

The numbers and sources

Effect: A review of over 20 studies with about 360 subjects reported increases in resting metabolic rate and modest weight loss over 6 to 12 weeks with bitter orange products, alone or in combinations. About two-thirds of overweight subjects also took 132 to 528 mg of caffeine a day [4].
Population: Over 50% overweight or obese adults, synephrine 10 to 53 mg a day for up to 12 weeks [4].
Sources: [4]
Caveats: The authors were consultants for Nutratech, Inc., which markets bitter orange extracts, and Nutratech supplied some unpublished reports [4]. Most studies tested combinations, not synephrine alone.

LimitedStudied in men

Yohimbine and body fat

One small trial found body fat fell from 9.3% to 7.1% over 21 days in male soccer players, with no change in performance.

The numbers and sources

Effect: In 20 professional soccer players, 20 mg a day for 21 days lowered body fat percentage from 9.3% to 7.1%, with no change in body mass, muscle mass or any performance test [5].
Population: Top-level male soccer players [5].
Sources: [5]
Caveats: One small trial.

Not shown to help1

Moderate

Weight and fat loss with thermogenic supplements

Not shown to lower body mass or fat mass in adults with BMI above 24.9 across 21 studies. The trend was smaller than effects reported for diet.

The numbers and sources

Effect: Across 21 studies longer than 8 weeks, thermogenic supplements trended toward lower body mass and fat mass, but the 95% confidence intervals crossed zero. Compared with previously reported effects of diet or diet plus exercise, supplement effects were smaller [1].
Population: Adults with BMI above 24.9 [1].
Sources: [1]
Caveats: Products and ingredients varied widely. The comparison with diet and exercise used effect sizes from other studies.

Doses studied

Synephrine trials used 10 to 53 mg a day, often with 132 to 528 mg of caffeine [4]. Single-product synephrine safety studies used about 49 mg as one dose or twice a day for 60 days [6][7]. The yohimbine trial used 20 mg a day in two doses [5]. Yohimbine supplements tested in the US contained none to 12.1 mg per labeled serving [8].

Risks

Reported risks3

Strong

Ephedra side effects and the 2004 FDA rule

Shown to raise psychiatric, autonomic and gut symptoms and palpitations 2.2 to 3.6 times. FDA declared ephedrine alkaloid supplements adulterated in 2004.

The numbers and sources

Effect: Across 50 trials, ephedra or ephedrine raised the odds of psychiatric, autonomic or gastrointestinal symptoms and heart palpitations by 2.2 to 3.6 times [2]. FDA declared supplements containing ephedrine alkaloids adulterated because they present an unreasonable risk of illness or injury, citing their pharmacology, the peer-reviewed literature and reported adverse events. The rule took effect April 12, 2004 [3].
Population: Controlled trials and case reports [2]; US market [3].
Sources: [2], [3]
Caveats: Trial data were too sparse to assess events rarer than 1 per 1,000 [2].

Limited

Yohimbine poisonings and mislabeling

Case reports: poison control yohimbine cases rose from 2000 to 2006, with gut distress, fast heart rate and anxiety. Most labeled amounts were inaccurate.

The numbers and sources

Effect: California Poison Control logged 238 symptomatic adult yohimbine cases from 2000 to 2006, rising from 1.8 to 8.0 per 10,000 adult exposures. Common effects were gastrointestinal distress (46%), tachycardia (43%), anxiety or agitation (33%) and hypertension (25%). Yohimbine exposures had higher odds of a severe outcome than the average exposure (OR 5.81, 95% CI 4.43 to 7.64) [9]. Of 49 US yohimbe supplements, 11 listed a yohimbine quantity, and most of those were inaccurate (23% to 147% of label) [8].
Population: Poison control cases [9]; products from seven major US retailers [8].
Sources: [8], [9]
Caveats: Poison center data capture only reported cases. 9.2% of cases involved weight loss use [9].

Moderate

Hidden drugs in weight loss products

FDA identified 317 adulterated weight loss supplements from 2007 to 2016, 84.9% containing sibutramine, a drug withdrawn from the market.

The numbers and sources

Effect: From 2007 to 2016, FDA identified 317 adulterated weight loss supplements, 84.9% containing sibutramine, a drug withdrawn from the market [11]. Weight loss or energy products were implicated in 71.8% of supplement-related emergency visits for palpitations, chest pain or tachycardia in the US [12].
Population: FDA Tainted Products database [11]; 63 US emergency departments, 2004 to 2013 [12].
Sources: [11], [12]
Caveats: FDA warnings capture products sampled, not the whole market.

Mixed results1

Limited

Synephrine: trials and case reports disagree

Small trials found no heart rate or blood pressure rise with bitter orange alone. Case reports describe chest pain, arrhythmias and stroke after multi-ingredient products.

The numbers and sources

Effect: In 18 healthy adults, a single 49 mg dose did not change heart rate, systolic pressure or ECG, and diastolic pressure fell 4.5 mmHg at 60 minutes [6]. In 25 per group over 60 days at about 49 mg twice a day, blood pressure and blood tests did not change [7]. Case reports describe 35 patients with chest pain, palpitations, syncope, ischemic heart disease, arrhythmias or stroke after synephrine-containing products, 5 left disabled or on medication [10].
Population: Healthy adults [6][7]; case reports [10].
Sources: [6], [7], [10]
Caveats: The trials tested bitter orange alone, while case reports usually involved multi-ingredient products with caffeine [10]. Authors of both trials also authored the Nutratech-linked review [4][6][7].

Claims tested and not found

That thermogenic supplements produce meaningful fat loss. Pooled confidence intervals for body mass and fat mass crossed zero [1].
That yohimbine improves sports performance. Strength, jump, sprint and agility tests did not change in soccer players [5].

Sources

Clark JE, Welch S. Comparing effectiveness of fat burners and thermogenic supplements to diet and exercise for weight loss and cardiometabolic health: Systematic review and meta-analysis. Nutrition and Health. 2021. https://doi.org/10.1177/0260106020982362
Shekelle PG, Hardy ML, Morton SC, et al. Efficacy and Safety of Ephedra and Ephedrine for Weight Loss and Athletic Performance: A Meta-analysis. JAMA. 2003. https://doi.org/10.1001/jama.289.12.1537
U.S. Food and Drug Administration. Final Rule Declaring Dietary Supplements Containing Ephedrine Alkaloids Adulterated Because They Present an Unreasonable Risk. Federal Register 69(28):6788. 2004. https://www.govinfo.gov/content/pkg/FR-2004-02-11/html/04-2912.htm
Stohs SJ, Preuss HG, Shara M. A Review of the Human Clinical Studies Involving Citrus aurantium (Bitter Orange) Extract and its Primary Protoalkaloid p-Synephrine. International Journal of Medical Sciences. 2012. https://doi.org/10.7150/ijms.4446
Ostojic SM. Yohimbine: The Effects on Body Composition and Exercise Performance in Soccer Players. Research in Sports Medicine. 2006. https://doi.org/10.1080/15438620600987106
Shara M, Stohs SJ, Mukattash TL. Cardiovascular Safety of Oral p-Synephrine (Bitter Orange) in Healthy Subjects: A Randomized Placebo-Controlled Cross-over Clinical Trial. Phytotherapy Research. 2016. https://doi.org/10.1002/ptr.5590
Kaats GR, Miller H, Preuss HG, et al. A 60 day double-blind, placebo-controlled safety study involving Citrus aurantium (bitter orange) extract. Food and Chemical Toxicology. 2013. https://doi.org/10.1016/j.fct.2013.01.013
Cohen PA, Wang Y, Maller G, et al. Pharmaceutical quantities of yohimbine found in dietary supplements in the USA. Drug Testing and Analysis. 2015. https://doi.org/10.1002/dta.1849
Kearney T, Tu N, Haller C. Adverse Drug Events Associated with Yohimbine-Containing Products: A Retrospective Review of the California Poison Control System Reported Cases. Annals of Pharmacotherapy. 2010. https://doi.org/10.1345/aph.1p060
de Jonge MLL, Kieviet LC, Sierts M, et al. Review of Case Reports on Adverse Events Related to Pre-workout Supplements Containing Synephrine. Cardiovascular Toxicology. 2023. https://doi.org/10.1007/s12012-022-09777-z
Tucker J, Fischer T, Upjohn L, et al. Unapproved Pharmaceutical Ingredients Included in Dietary Supplements Associated With US Food and Drug Administration Warnings. JAMA Network Open. 2018. https://doi.org/10.1001/jamanetworkopen.2018.3337
Geller AI, Shehab N, Weidle NJ, et al. Emergency Department Visits for Adverse Events Related to Dietary Supplements. New England Journal of Medicine. 2015. https://doi.org/10.1056/nejmsa1504267

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