F tier · safety concern
There is no approved human indication in the US or UK to grade against — the only US approval is veterinary — and the largest body of human data on clenbuterol is a poisoning literature, not an efficacy literature.
clenbuterol
Clenbuterol is a long-acting beta-2 adrenoceptor agonist that the FDA has approved only for airway obstruction in horses; it has never been approved for human use in the United States. Where human data exist in the peer-reviewed record they come predominantly from poison-centre case series and food-contamination outbreaks rather than from controlled efficacy trials.
the explanation
Clenbuterol is an asthma-type drug for horses that was never cleared for people in the US or UK. Most of what doctors have published about it in humans describes poisonings — racing hearts, shaking, and low potassium — rather than benefits.
regulatory status
Not approved for human use (US/UK); veterinary only
The DEA drug and chemical information sheet states clenbuterol is not controlled under the Controlled Substances Act but has never been approved by the FDA for human use; the only FDA approval is Ventipulmin Syrup (1998) for airway obstruction in horses. Some other countries license it as a human bronchodilator for bronchial asthma. It is prohibited in sport.
how it works · proposed mechanism
Clenbuterol is a selective agonist at the beta-2 adrenoceptor, a Gs-coupled receptor expressed on airway smooth muscle, skeletal muscle and cardiac tissue.
Beta-2 receptor activation
Binding to the beta-2 adrenoceptor activates adenylyl cyclase and raises intracellular cyclic AMP. In airway smooth muscle this drives relaxation and bronchodilation, the basis of its veterinary licence.
Skeletal muscle repartitioning
In livestock and rodent models beta-2 agonism increases skeletal muscle protein accretion and reduces adipose mass, an effect that made beta-agonists commercially attractive as growth promoters. Whether this translates to healthy humans at tolerated exposures has not been established in controlled trials.
Cardiac and electrolyte effects
Beta-2 receptors are also present on cardiac myocytes and drive Na+/K+-ATPase activity, shifting potassium and phosphate intracellularly. This underlies the tachycardia, hypokalaemia and hypophosphataemia described in human poisoning reports.
what’s reported
evidence shape
6 sourcesThe composition of the cited literature by study type. This describes the shape of the evidence, not its quality — and it does not by itself set the grade.
⚠ the catch
Clenbuterol is marketed to consumers as a fat-loss agent, but the human literature indexed in PubMed contains essentially no controlled trials of body composition in healthy people — the repartitioning data are from livestock and rodents, and the one human muscle study was in heart-failure patients on ventricular assist devices. The evidence that does exist in humans is a toxicity literature, so nothing here transfers to the physique-enhancement use it is sold for.
key published findings
- Human (poison-centre case series): Spiller and colleagues reviewed 13 clenbuterol users across two poison centres, 11 of them intentional misuse for weight loss or bodybuilding; clinical effects included tachycardia, widened pulse pressure, hypokalaemia, hyperglycaemia, ST changes on ECG, elevated troponin and elevated CPK, with effects persisting beyond 24 hours and evidence of myocardial injury in 2 patients. One patient had a serum concentration of 2983 pg/mL.
- Human (outbreak epidemiology): Barbosa and colleagues described four clenbuterol food-poisoning incidents in Portugal affecting 50 people, traced to lamb and bovine meat and liver, presenting with gross tremors of the extremities, tachycardia, nausea, headaches and dizziness.
- Human trial (disease population, not healthy): George and colleagues studied clenbuterol during left ventricular assist device support and reported body weight rising from 75.5 ± 17.9 to 79.2 ± 25.1 kg, lean mass from 21.1 ± 8.9 to 23.6 ± 9.7 kg, and quadriceps maximal voluntary contraction from 37.0 ± 15.7 to 45.8 ± 20.6 kg (all p < 0.05) — a heart-failure cohort, not healthy volunteers.
- Rodent: chronic clenbuterol administration induced hypertrophy of both the latissimus dorsi muscle and the heart in rats, with accompanying molecular and phenotypic changes (Circulation, 1995).
- Rodent: Burniston and colleagues showed clenbuterol produced dose-dependent myocyte death in male Wistar rats, with peak apoptosis of 5.8 ± 2.0% and peak necrosis of 7.4 ± 1.7% in soleus muscle; at 12 hours, 73% of damaged myocytes were necrotic only and none were purely apoptotic, and repeated administration over 8 days produced cumulative myocyte death.
limitations of the evidence
- No randomised controlled trials of clenbuterol for body composition in healthy humans are indexed, so efficacy for the marketed use is untested rather than merely weak.
- Poison-centre case series are self-selected toward severe presentations and cannot establish an incidence rate; several reported cases also involved co-ingested thyroid hormone and anabolic steroids, confounding attribution.
- Because no human pharmaceutical product is marketed in the US or UK, material reaching consumers is of unverified identity, purity and content, so exposure in case reports is often unquantified.
documented safety signals
- Tachycardia, palpitations, chest pain and widened pulse pressure are the most consistently reported findings in human exposure reports.
- Supraventricular tachycardia and atrial fibrillation have been reported after acute overdose.
- Elevated troponin and elevated creatine phosphokinase consistent with myocardial injury were documented in poison-centre cases.
- Hypokalaemia, hypophosphataemia and hyperglycaemia arise from beta-2-mediated intracellular electrolyte shifting; potassium replacement has been required.
- The long 26-39 hour persistence means adverse effects are prolonged, with clinical effects persisting beyond 24 hours in reported cases.
- Tremor, tachypnoea, nervousness, headache, dizziness, nausea and vomiting are recurrent across both misuse cases and contaminated-meat outbreaks.
- Animal studies document cardiac hypertrophy and dose-dependent myocyte apoptosis and necrosis; there is no human safety dataset establishing that these do not occur.
- Documented food-poisoning outbreaks from clenbuterol residues in meat and liver have affected dozens of people at a time across multiple countries, prompting proposed clinical guidelines for clenbuterol food poisoning.
- Reported misuse cases frequently involve co-ingestion with thyroid hormone and anabolic steroids; the FDA liothyronine label specifically warns that thyroid hormone toxicity risk is amplified in combination with sympathomimetic amines.
- There is no approved human product in the US or UK, so consumers rely on unregulated supply chains with attendant dose and contaminant uncertainty.
- Prohibited in sport, with the long half-life making inadvertent and deliberate exposure difficult to distinguish.
identity
| full name | Clenbuterol hydrochloride |
| category | Thyroid & Stimulant |
| modality | small molecule |
| formula | C12H18Cl2N2O |
| molar mass | 277.19 g/mol |
| cas | 37148-27-9 |
| half-life | ~26-39 h |
laboratory handling
| storage | Solid clenbuterol hydrochloride is stored dry at room temperature and protected from light. No human pharmaceutical formulation is marketed in the US or UK, so no approved storage instructions exist for human product. |
| solubility | The hydrochloride salt is water-soluble and soluble in ethanol; the free base is lipophilic, consistent with its wide tissue distribution and long persistence. |
| co-studied with | The FDA liothyronine (Cytomel) label explicitly warns that in euthyroid patients larger thyroid hormone doses risk serious toxicity, particularly when combined with sympathomimetic amines — the exact combination described in several clenbuterol poison-centre reports, where co-ingestants included thyroid hormone and anabolic steroids. This is documented as a hazard signal, not a regimen. |
Handling information describes laboratory practice for a research reagent. It is not a preparation guide for use in humans.
the receipts
6 citedUS DEA Diversion Control Division · 2013 · official
Substance Abuse · 2013 · observational
Food Additives and Contaminants · 2005 · observational
Journal of Heart and Lung Transplantation · 2006 · observational
Circulation · 1995 · preclinical
Muscle & Nerve · 2005 · preclinical
others in Thyroid & Stimulant