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Clenbuterol (Clenbutérol) — long-acting beta-2 adrenergic agonist
A long-acting beta-2 adrenergic agonist used illicitly for fat loss / 'cutting' and as a livestock repartitioning agent — it is NOT a dietary supplement and is not approved for human use in the US (it is a veterinary drug, and an asthma medicine in a few countries). The 'fat-burner' reputation rests entirely on ANIMAL repartitioning studies (more lean mass, less fat in pigs and cattle); there is no human fat-loss RCT. The real human record is the harm: poisoning outbreaks from clenbuterol-tainted meat and recreational-overdose case series with tachycardia, severe hypokalemia, arrhythmia, and myocardial injury, plus cardiac hypertrophy in animals. It is WADA-banned. The cardiac toxicity, not the fat loss, is the story.
Research compound — not a dietary supplement
Clenbuterol is a research compound, not a regulated dietary supplement. It is sold for research or off-label use. The evidence below is largely preclinical (animal and in-vitro) or early-stage, so no evidence score is assigned. This page is provided for transparency and education — it is not a recommendation to use. Consult a qualified healthcare provider, and be aware that purity, dosing, and legal status vary by jurisdiction.
What the evidence says
Most Clenbuterol studies are mechanism or observational rather than RCTs that measure a clinical effect — keep findings provisional.
Most evidence is from high-quality randomised trials published 1977–2026 with a typical study size of 113 participants.
Based on 30 studies · 22 RCTs · 127 total participants
Confidence
Moderate confidenceBy outcome
Clenbuterol's fat-loss / 'cutting' reputation rests entirely on ANIMAL repartitioning and anti-catabolic studies (leaner pigs, cattle, and rodents) — there is no human fat-loss RCT. The human evidence is almost all toxicity: poisoning outbreaks from clenbuterol-tainted meat and recreational-overdose case series with tachycardia, severe hypokalemia, arrhythmia, and myocardial injury, plus cardiac hypertrophy in animals. It is not approved for human use in the US, is not a dietary supplement, and is WADA-banned — the cardiac toxicity, not the fat loss, defines the verdict, so it scores low.
26 rigorous studies
25 randomized trials · 0 meta-analyses · 1 systematic reviews
Our evidence rating for Clenbuterol is accountable to this entire body of rigorous research indexed in PubMed — not a hand-picked subset.
PubMed · as of Jul 2026
2 trials ongoing or recruiting · 6 completed on ClinicalTrials.gov
3 of the completed trials have posted results
Registered trials show research momentum for Clenbuterol, not proof of effect — a registration is a plan, and posted results are sponsor-reported, not peer-reviewed. They are never counted toward the evidence rating above.
Browse these trials on ClinicalTrials.govClinicalTrials.gov · as of Aug 2026
Clenbuterol is a long-acting, lipophilic beta-2 adrenergic agonist — pharmacologically a bronchodilator like salbutamol, but with a much longer duration of action.
It is used as an asthma/COPD drug in a handful of countries (e.g. as Spiropent) and as a veterinary bronchodilator (Ventipulmin), but it is NOT approved for human use in the United States and is not a dietary-supplement ingredient anywhere.
It is famous in bodybuilding and 'cutting' culture as a fat-loss agent and is banned by the World Anti-Doping Agency. The honest core of this entry is the gap between where the efficacy evidence comes from and where it is being used.
Clenbuterol's reputation as a body-recomposition drug rests almost entirely on ANIMAL 'repartitioning' research: in pigs, cattle, and rodents, beta-2 agonists shift nutrient partitioning toward skeletal-muscle protein accretion and away from fat, increasing carcass lean mass and decreasing fat — which is exactly why it was used illegally as a livestock growth promoter and why it caused human poisoning outbreaks from contaminated meat.
The mechanistic animal work is real: clenbuterol's anabolic effect on skeletal muscle is mediated by beta-2-adrenoceptor activation (Choo 1992), and it potently antagonizes muscle protein wasting in cancer-cachexia rats by suppressing the ATP-ubiquitin proteolytic pathway (Costelli 1995).
But there is no human randomized controlled trial demonstrating fat loss or body recomposition — the leap from 'leaner pigs' to 'leaner humans' has never been made in a controlled human efficacy study. What human data DO exist are overwhelmingly about toxicity.
Clenbuterol-tainted meat (veal liver) caused a 113-case poisoning outbreak in Catalonia, Spain, with tachycardia, tremor, myalgia, and nervousness (Salleras 1995), one of several such outbreaks across Europe.
Recreational and adulterated-drug exposure is worse: a case series of clenbuterol toxicity from adulterated heroin described patients with chest pain and palpitations, a median heart rate of 120, a serum potassium nadir of 2.5 mEq/L (severe hypokalemia), elevated lactate, and troponin elevation with myocardial injury despite clean coronary arteries (Hieger 2016).
A 2023 PRISMA systematic review of adverse events among athletes found the dominant harms were cardiac — supraventricular tachycardia, atrial fibrillation, myocardial injury, myocardial infarction, cardiomyopathy — and concluded that the lack of evidence for performance-enhancing effects combined with serious, sometimes fatal toxicity questions any use of the drug (Kumari 2023).
On the cardiac-structure side, clenbuterol induces cardiac hypertrophy alongside skeletal-muscle hypertrophy in rats (Petrou 1995), a structural trade-off that mirrors the human arrhythmia signal.
The honest summary: a beta-2 agonist with genuine animal repartitioning and anti-catabolic data, NO human fat-loss RCT, a real and well-documented cardiac-toxicity and poisoning record, WADA-banned, and not approved for human use in the US. It is not a dietary supplement and not a longevity drug.
The cardiac risk is the headline, so it scores low.
Clenbuterol is a long-acting, lipophilic agonist of the beta-2 adrenergic receptor — the same receptor class as asthma bronchodilators, but with a far longer duration of action. Sustained beta-2 stimulation is what drives both its proposed metabolic effects and its cardiovascular and metabolic toxicity (tachycardia, tremor, hypokalemia).
In livestock and rodents, beta-2 agonism shifts nutrient partitioning toward skeletal-muscle protein accretion and away from fat — increasing carcass lean mass and reducing fat. Clenbuterol also suppresses the ATP-ubiquitin proteolytic pathway, antagonizing muscle protein breakdown. This repartitioning is the animal basis for the human 'cutting' reputation, demonstrated in pigs, cattle, and rats — not in human trials.
The same beta-2 stimulation that drives muscle repartitioning also produces tachycardia and arrhythmia, drives potassium intracellularly to cause hypokalemia, and — at sustained high exposure in animals — induces cardiac hypertrophy. In human overdose and tainted-meat poisoning this manifests as tachycardia, severe hypokalemia, myocardial injury, and arrhythmia.
How Clenbuterol works — from molecular targets to health outcomes. Click an edge to see supporting research.This visualization is in beta — pathways are being refined and expanded.
Avoid — there is no human fat-loss RCT, it is not approved for human use in the US, and the human record is dominated by cardiac toxicity and hypokalemia.
Avoid absolutely — clenbuterol causes tachycardia, arrhythmia, and myocardial injury and can precipitate a cardiac event.
Avoid absolutely — WADA-prohibited; detected in doping control and a documented cause of sanctions.
Avoid entirely — a beta-2 agonist with tocolytic activity, unstudied for this use, with cardiac and metabolic toxicity.
Additive beta-adrenergic stimulation sharply raises the risk of tachycardia, arrhythmia, and cardiac toxicity. Stacking clenbuterol with stimulants is a recurring feature of the overdose case reports.
Clenbuterol drives potassium intracellularly, causing hypokalemia; combining it with potassium-wasting drugs deepens the deficit and worsens arrhythmia risk.
Beta-blockers are used to counter clenbuterol toxicity, but the interaction is pharmacologically opposed and uncharacterised for chronic co-use; do not combine outside emergency care.
Clenbuterol-induced hypokalemia potentiates digoxin toxicity and the proarrhythmic effect of QT-prolonging drugs.
Tip: There is no safe self-managed mitigation. Tachycardia, supraventricular tachycardia, atrial fibrillation, myocardial injury and even myocardial infarction are documented in overdose and athlete case series; cardiac complications are the dominant harm. Seek emergency care for chest pain, palpitations, or breathlessness.
Tip: Clenbuterol drives potassium into cells; a poisoning case series recorded a potassium nadir of 2.5 mEq/L. Hypokalemia worsens arrhythmia risk and requires medical correction.
Tip: Tremor, nervousness, tachycardia, myalgia, and headache dominated the tainted-meat poisoning outbreaks; symptoms can last hours to days. There is no protocol that makes this safe at fat-loss doses.
Tip: Clenbuterol has poisoned people via contaminated meat and adulterated street drugs. Grey-market material has no identity/purity/dose guarantees, and clenbuterol contamination has caused mass-poisoning outbreaks.
Clenbuterol has an evidence score of 3/10 — emerging evidence based on 8 indexed studies. A long-acting beta-2 adrenergic agonist used illicitly for fat loss / 'cutting' and as a livestock repartitioning agent — it is NOT a dietary supplement and is not approved for human use in the US (it is a veterinary drug, and an asthma medicine in a few countries). The 'fat-burner' reputation rests entirely on ANIMAL repartitioning studies (more lean mass, less fat in pigs and cattle); there is no human fat-loss RCT. The real human record is the harm: poisoning outbreaks from clenbuterol-tainted meat and recreational-overdose case series with tachycardia, severe hypokalemia, arrhythmia, and myocardial injury, plus cardiac hypertrophy in animals. It is WADA-banned. The cardiac toxicity, not the fat loss, is the story. Representative study: PMID 37062796.
The commonly studied dose of Clenbuterol is No legitimate or recommended dose — clenbuterol is not approved for human use in the US, is not a dietary supplement, and carries documented cardiac toxicity and a sport ban. We do NOT provide a fat-loss or 'cutting' dosing protocol. For context only, illicit bodybuilding use is reported in the microgram range (commonly cited as ~20–120 mcg/day, escalating), and athlete case reports span 20 mcg to 30 mg — wide, unvalidated, and associated with the toxicity in those very reports. That is not a recommended regimen.. Individual needs vary — start at the lower end of the range and adjust based on how you respond.
Timing is flexible for Clenbuterol — consistent daily use matters more than the time of day. There is no validated or endorsed human fat-loss dosing schedule.
Clenbuterol should be used with caution — talk to a healthcare provider before taking it. The most commonly reported side effects are cardiac toxicity (tachycardia, arrhythmia, myocardial injury), severe hypokalemia, tremor, palpitations, nervousness. Use caution if any of these apply to you: Anyone with cardiovascular disease, arrhythmia, hypertension, or hyperthyroidism — clenbuterol causes tachycardia, arrhythmia, and myocardial injury; Not an approved human medicine in the US and not a regulated dietary supplement — a veterinary/foreign asthma drug misused for fat loss; do not self-source; Competitive or tested athletes — WADA-prohibited; will trigger an anti-doping violation.
Caffeine
Likely helpsBlocks adenosine receptors to boost alertness, reaction time, and endurance — one of the most proven ergogenic aids.
Berberine
Likely helpsActivates AMPK to regulate blood sugar, improve insulin sensitivity, and support lipid metabolism — comparable to metformin in some trials.
Epicatechin
Probably helpsDark chocolate flavanol with consistent human evidence for improved endothelial function and modest blood pressure reduction. Muscle-building claims are not supported in humans.
Orlistat
Mostly mechanism / observationalA gastrointestinal lipase inhibitor that blocks roughly 30% of dietary fat from being absorbed. It is the only weight-loss drug sold both on prescription (Xenical 120 mg) and over the counter (Alli 60 mg). The evidence base is large and old: across 16 long-term randomised trials it produces about 2.9 kg more weight loss than placebo, and in the 4-year XENDOS trial it cut new type 2 diabetes from 9.0% to 6.2%, though exploratory analysis found the effect was detectable ONLY in the ~21% of participants who already had impaired glucose tolerance. The trade-off is that the unabsorbed fat leaves in the stool — oily spotting, faecal urgency and steatorrhoea — and it measurably impairs fat-soluble vitamin absorption, which is what drives its real drug interactions (warfarin, levothyroxine, ciclosporin). Modest, unglamorous, and hard to stay on.
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Reviewed by Dr. Baher Al Hakim · Last reviewed June 2026 · evidence from 30 studies · how we score · editorial policy
This information is for educational purposes only. It is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement or medication.
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