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Clenbuterol

Updated · 21 sources · Editorial policy

Clenbuterol is a long-acting beta-2 adrenergic agonist, a drug in the same family as the asthma inhaler albuterol. In some countries it is sold as a tablet for chronic obstructive airway disease, and it is used in veterinary medicine as a bronchodilator for horses. It is not a steroid or a hormone.

In the United States, clenbuterol has never been approved for human use. FDA’s only approval is an oral syrup for horses, and federal law bans its use in food-producing animals. Outside medicine it is taken for fat loss and bodybuilding, mainly because of striking muscle and fat effects in livestock and rodents. Human evidence for those effects is thin, while poison-center reports of heart and metabolic toxicity are well documented.

This page summarizes what regulators, clinical studies and poison-center reports describe. It is for education only and is not medical advice.

Property Detail
Drug class Selective beta-2 adrenergic agonist (sympathomimetic), not a steroid
Human use Bronchodilator for chronic obstructive airway disease in some countries; EMA cites 10–20 µg twice daily
US approval None for humans; approved only as an oral syrup for horses (72.5 µg/mL)
Route Oral in humans; oral or injected in veterinary use
Half-life About 35 h in healthy volunteers
Plasma protein binding 89–98%
Food animals Extralabel use prohibited in US food-producing animals; EU bans beta-agonists in stock farming
US controlled status Not listed in the federal controlled-substance schedules
WADA status Prohibited at all times as an “other anabolic agent” (S1.2), with no threshold

What it is

Clenbuterol’s chemical name is 1-(4-amino-3,5-dichlorophenyl)-2-(tert-butylamino)ethanol: formula C₁₂H₁₈Cl₂N₂O, molecular weight 277.19 g/mol (PubChem). It is usually given as the hydrochloride salt (EMA summary report, 2000).

The European Medicines Agency’s veterinary committee describes it this way:

  • a direct-acting beta-2 sympathomimetic, meaning it switches on the same receptors adrenaline uses in the airways, heart, muscle and fat;
  • manufactured as a 50:50 racemic mixture of two mirror-image forms, with most of the activity in the levo (left-handed) form;
  • used in human medicine for chronic obstructive airway disease at 10–20 µg twice a day;
  • used in horses for airway disease and in cattle and horses to relax the uterus during birth (tocolysis).

Why it is called an anabolic

In animals, clenbuterol shifts nutrients from fat toward muscle, an effect often called “repartitioning.” In rats treated for 16–19 days, clenbuterol increased body protein gain by 50% and muscle protein synthesis by 34%, and enlarged the gastrocnemius (calf) muscle (Emery et al., Biosci Rep 1984). These are rat results. The doses and responses do not transfer directly to people, and the human data below are far more modest.

WADA lists clenbuterol under anabolic agents rather than with the other beta-2 agonists. The listing is a sports-regulation category. It does not by itself show a large muscle-building effect in humans.

Pharmacokinetics

The main human kinetic study gave healthy volunteers single oral doses of 20, 40 or 80 µg, then repeated twice-daily dosing (Yamamoto et al., J Pharmacobiodyn 1985):

Parameter Finding
Peak plasma level 0.1, 0.2 and 0.35 ng/mL after 20, 40 and 80 µg
Time to peak Within 2.5 h
Plasma half-life About 35 h
Steady state with twice-daily dosing Reached within 4 days
Steady-state level 0.2–0.3 ng/mL at 20 µg; 0.5–0.6 ng/mL at 40 µg
Plasma protein binding 89–98%
Unchanged drug in urine About 20% of the dose by 72 h

EMA reports that clenbuterol is well absorbed by mouth in humans and animals, peaks in blood 2–3 hours after an oral dose, spreads widely into tissues and crosses the placenta in several species. Excretion is mainly in urine as unchanged drug.

The long half-life matters for safety. Effects and toxicity can persist for a day or more after a dose, which matches the prolonged symptoms seen in poisonings.

Regulatory status

United States

  • Horses only. FDA regulations approve a clenbuterol syrup containing 72.5 µg/mL of clenbuterol hydrochloride for horses with airway obstruction. Dosing starts at 0.8 µg/kg twice daily and steps up to no more than 3.2 µg/kg, under veterinary prescription (21 CFR 520.452).
  • Banned in food animals. Clenbuterol is on FDA’s list of drugs prohibited from extralabel use in food-producing animals (21 CFR 530.41).
  • No human product. US poison-center researchers describe clenbuterol as approved in the US only for veterinary use in non-food animals (Spiller et al., Subst Abus 2013).
  • Not a scheduled drug. Clenbuterol does not appear in the DEA schedules of controlled substances (21 CFR Part 1308).

European Union

EU law (Council Directive 96/22/EC) prohibits beta-agonists in stock farming, apart from defined veterinary uses. EMA set an acceptable daily intake of 0.25 µg per person, based on a no-effect dose for airway effects in humans of 2.5 µg/day, and set residue limits for cattle and horse tissues and cow’s milk (EMA summary report, 2000).

Sport

On the 2026 WADA Prohibited List, clenbuterol is named under S1.2, other anabolic agents, and is banned in and out of competition. WADA states there is no threshold below which it is allowed, but accepts that low levels can sometimes come from contaminated meat. Each case is assessed individually (WADA Prohibited List).

What the research shows

There are no large or long-term controlled trials of clenbuterol for body composition in people. The human evidence consists of small physiology studies, one small surgical trial and pilot studies in rare muscle diseases.

Energy expenditure and metabolism in healthy men

Study Design and dose Main findings
Jessen et al., Drug Test Anal 2020 6 young men, single 80 µg dose, measured before and 140 min after Resting energy expenditure +21%; fat oxidation +39%; blood glucose +30%, insulin +130%, lactate +90%, fatty acids +180%; maximal knee-extensor torque −4%
van Beek et al., Nat Commun 2023 Randomized, double-blind, placebo-controlled crossover; 11 healthy men; 40 µg/day for 14 days Insulin-stimulated glucose disposal +13%; sleeping metabolic rate +10.2%; basal metabolic rate +5.6%; triglycerides −16.4%; no change in body weight, fat mass or lean mass

In the 2023 crossover trial, heart rate rose by about 11 beats/min after two weeks. Five of 11 men reported hand tremor, four had muscle aches or cramps, two felt anxious or restless and two had headaches. All side effects resolved after stopping, and there were no serious adverse events (PMC full text).

“Clenbuterol builds muscle like a steroid.” The clear muscle-building effects come from livestock and rodent studies. In the only randomized, placebo-controlled human trial that measured body composition, two weeks at 40 µg/day changed neither lean mass nor fat mass. The single-dose study found higher muscle mTOR signaling, a marker of protein-synthesis activity, but did not measure muscle growth.

Recovery after knee surgery

A double-blind randomized trial gave clenbuterol or placebo to 20 healthy men for 4 weeks after open meniscus surgery, followed by a 2-week washout (Maltin et al., Clin Sci 1993). Strength in the operated leg recovered faster with clenbuterol. However, absolute strength did not differ significantly between the groups.

Neuromuscular diseases

Clenbuterol has been tested as an add-on in rare muscle and motor neuron diseases:

Condition Study Dose and duration Findings
Late-onset Pompe disease (on enzyme therapy) Randomized, double-blind, placebo-controlled phase I/II; 13 enrolled, 11 completed (Koeberl et al., Mol Ther 2018) 52 weeks 6-minute walk +16 m (p = 0.08); maximum inspiratory pressure +8%; muscle glycogen −50%; mild CK rises, cramps and tremor
Spinal and bulbar muscular atrophy Open-label, 20 patients, 16 completed (Querin et al., Neurology 2013) 0.04 mg/day for 12 months 6-minute walk distance and forced vital capacity increased (p under 0.001); serum CK rose; no serious cardiac effects
Amyotrophic lateral sclerosis (ALS) Open-label, 25 patients (Li et al., J Clin Neuromuscul Dis 2023) 40 µg daily, raised to 80 µg twice daily Slower decline than before treatment; 14 withdrew early, 13 of them because of side effects

The open-label studies had no placebo group, so they cannot show that clenbuterol caused the changes. A 2026 expert review concluded it could not endorse clenbuterol for ALS. It noted methodological flaws in the supportive studies and frequent dropouts from side effects (Burgess et al., ALS Untangled 2026).

Heart failure with a ventricular assist device

In a single-center study, 15 patients with severe nonischemic heart failure received a left ventricular assist device and a drug regimen. Clenbuterol was added after the heart had shrunk back toward normal size, to try to prevent heart muscle atrophy. Eleven of the 15 recovered enough to have the device removed (Birks et al., NEJM 2006). There was no control group, and clenbuterol was one part of a multidrug regimen, so its own contribution cannot be separated.

Side effects and risks

At medical doses

EMA reports that human clinical studies generally rated clenbuterol as well tolerated at therapeutic doses. The adverse effects described were restlessness, palpitations and muscle tremor (EMA summary report, 2000). In the 2023 trial, 40 µg/day raised heart rate by about 11 beats/min and caused tremor in nearly half of participants.

Poisoning and misuse

Most serious harm reported in people comes from doses far above medical ones, taken for fat loss or bodybuilding, or from contaminated drugs and food.

  • Australian poison center, 2004–2012. Calls about clenbuterol rose from 3 in 2008 to 27 in 2012, with 63 exposures in all. At least 53 patients (84%) were hospitalized. The most common reasons for use were bodybuilding and slimming. The most common effects were tachycardia (a fast heart rate) in 24 patients, digestive upset in 16 and tremor in 11, and one 21-year-old man had a cardiac arrest (Brett et al., Med J Aust 2014).
  • US poison centers. In 13 users, 11 had taken clenbuterol for weight loss or bodybuilding. Effects included tachycardia, low potassium, high blood sugar, ECG changes, raised troponin (a marker of heart muscle injury) and chest pain. Effects lasted more than 24 hours, and two patients had evidence of heart muscle injury. Some had also taken T3 and anabolic steroids (Spiller et al., Subst Abus 2013).
  • Adulterated heroin, US East Coast. In 2005, CDC described 26 people with atypical reactions after heroin use, 24 of whom were hospitalized. Clenbuterol was detected in 8 (CDC, MMWR 2005). A poison-center study of the outbreak found 34 emergency presentations. Patients had tachycardia, low blood pressure, high blood sugar, low potassium and raised lactate, and six had biochemical evidence of heart muscle injury (Hoffman et al., Ann Emerg Med 2008).
  • Contaminated meat. In Catalonia, Spain, in 1992, 113 people were poisoned, mostly linked to eating veal liver. Over half had nervousness, tachycardia, muscle tremor, muscle pain and headache. Symptoms started 15 minutes to 6 hours after eating and lasted 90 minutes to 6 days. No one died (Salleras et al., Public Health Rep 1995).

The heart

Cardiac toxicity is the most consistent concern across species. In animal toxicity studies reviewed by EMA, rats developed dose-related heart muscle lesions. In dogs, tachycardia appeared at the lowest dose tested, and heart muscle necrosis (tissue death) occurred at higher doses. In people, the poison-center series above describe tachycardia, ECG changes, raised troponin and a cardiac arrest.

Metabolic effects

Low potassium, high blood sugar and raised lactate (lactic acidosis) were common findings in the poisoning reports. In the single-dose study in healthy men, glucose rose by 30% and insulin by 130% within hours.

Pregnancy

Clenbuterol crosses the placenta in animals. In rats and rabbits it caused birth defects only at doses that were also toxic to the mother. In rat birth studies, high doses increased pup deaths (EMA summary report, 2000). EMA mentions clinical studies in pregnant women with premature labor but gives no detailed safety results from them.

Key takeaways

  • A beta-2 agonist, not a steroid. Clenbuterol is an asthma-type drug used at 10–20 µg twice daily in some countries. In the US it is approved only for horses.
  • Long half-life. At about 35 hours, effects and toxicity can last more than a day after a dose.
  • Muscle claims rest mostly on animal data. Rats and livestock show clear repartitioning. In a 2-week randomized human trial, clenbuterol raised metabolic rate by 5–10% but did not change lean or fat mass.
  • The heart is the main risk. Tachycardia, low potassium, high blood sugar, ECG changes and heart muscle injury recur across poison-center reports, including a cardiac arrest in a young man.
  • Banned in food animals and in sport. US and EU law prohibit it in livestock. WADA bans it at all times with no threshold, although contaminated meat can cause low-level positives.

Sources

  1. European Agency for the Evaluation of Medicinal Products, Committee for Veterinary Medicinal Products. Clenbuterol: summary report (2). EMEA/MRL/723/99-FINAL. February 2000. EMA.
  2. National Center for Biotechnology Information. Clenbuterol, PubChem CID 2783. PubChem.
  3. Code of Federal Regulations. 21 CFR 520.452, Clenbuterol syrup. eCFR.
  4. Code of Federal Regulations. 21 CFR 530.41, Drugs prohibited for extralabel use in animals. eCFR.
  5. Code of Federal Regulations. 21 CFR Part 1308, Schedules of controlled substances. eCFR.
  6. World Anti-Doping Agency. The 2026 Prohibited List and Prohibited List Q&A. WADA.
  7. Yamamoto I, Iwata K, Nakashima M. Pharmacokinetics of plasma and urine clenbuterol in man, rat, and rabbit. J Pharmacobiodyn. 1985;8(5):385–391. PubMed.
  8. Emery PW, Rothwell NJ, Stock MJ, Winter PD. Chronic effects of beta 2-adrenergic agonists on body composition and protein synthesis in the rat. Biosci Rep. 1984;4(1):83–91. PubMed.
  9. Jessen S, Solheim SA, Jacobson GA, et al. Beta(2)-adrenergic agonist clenbuterol increases energy expenditure and fat oxidation, and induces mTOR phosphorylation in skeletal muscle of young healthy men. Drug Test Anal. 2020;12(5):610–618. PubMed.
  10. van Beek SMM, Bruls YMH, Vanweert F, et al. Effect of β2-agonist treatment on insulin-stimulated peripheral glucose disposal in healthy men in a randomised placebo-controlled trial. Nat Commun. 2023;14(1):173. PubMed; PMC.
  11. Maltin CA, Delday MI, Watson JS, et al. Clenbuterol, a beta-adrenoceptor agonist, increases relative muscle strength in orthopaedic patients. Clin Sci (Lond). 1993;84(6):651–654. PubMed.
  12. Koeberl DD, Case LE, Smith EC, et al. Correction of biochemical abnormalities and improved muscle function in a phase I/II clinical trial of clenbuterol in Pompe disease. Mol Ther. 2018;26(9):2304–2314. PubMed.
  13. Querin G, D’Ascenzo C, Peterle E, et al. Pilot trial of clenbuterol in spinal and bulbar muscular atrophy. Neurology. 2013;80(23):2095–2098. PubMed.
  14. Li X, Koeberl DD, Lutz MW, Bedlack R. Clenbuterol treatment is safe and associated with slowed disease progression in a small open-label trial in patients with amyotrophic lateral sclerosis. J Clin Neuromuscul Dis. 2023;24(4):214–221. PubMed.
  15. Burgess A, Allen O, Barkhaus P, et al. ALS untangled #83: clenbuterol. Amyotroph Lateral Scler Frontotemporal Degener. 2026 (online ahead of print):1–8. PubMed.
  16. Birks EJ, Tansley PD, Hardy J, et al. Left ventricular assist device and drug therapy for the reversal of heart failure. N Engl J Med. 2006;355(18):1873–1884. PubMed.
  17. Brett J, Dawson AH, Brown JA. Clenbuterol toxicity: a NSW poisons information centre experience. Med J Aust. 2014;200(4):219–221. PubMed.
  18. Spiller HA, James KJ, Scholzen S, Borys DJ. A descriptive study of adverse events from clenbuterol misuse and abuse for weight loss and bodybuilding. Subst Abus. 2013;34(3):306–312. PubMed.
  19. Centers for Disease Control and Prevention. Atypical reactions associated with heroin use: five states, January–April 2005. MMWR Morb Mortal Wkly Rep. 2005;54(32):793–796. PubMed.
  20. Hoffman RS, Kirrane BM, Marcus SM; Clenbuterol Study Investigators. A descriptive study of an outbreak of clenbuterol-containing heroin. Ann Emerg Med. 2008;52(5):548–553. PubMed.
  21. Salleras L, Domínguez A, Mata E, et al. Epidemiologic study of an outbreak of clenbuterol poisoning in Catalonia, Spain. Public Health Rep. 1995;110(3):338–342. PubMed.