Clenbuterol
Selective β2-adrenoceptor agonist, EU veterinary Rx for equine asthma. Off-label bodybuilding use for fat loss + muscle sparing.
The links above are labelled third-party affiliate links. MolekulaX does not sell and does not verify product quality.
WHAT IS CLENBUTEROL?
Detailed overview
Clenbuterol is a long-acting selective β2-adrenoceptor agonist originally synthesized in the 1970s as a bronchodilator. In the EU (including Hungary) it is veterinary Rx for equine asthma (Ventipulmin), and in a few countries (Bulgaria, China) human asthma Rx; in the US it is NOT approved for human medical use. Off-label bodybuilding use selectively hypertrophies type 2 (fast-glycolytic) muscle fibers and drives thermogenesis via β2-mediated UCP1 activation in brown fat. Its half-life is extremely long (~35-40 h), complicating dosing and increasing cardiotoxic accumulation risk.
Mechanism
Selective β2-adrenoceptor agonist
Half-life
35-40 hours
Onset
15-30 min
Legal status
EU: vet-Rx (equine asthma). HU: vet-Rx. USA: not approved for humans.
Data console
Lab data
Safety
Side effects, stop signs, contraindications
Side effects · 8
- Tachycardia and palpitations: β2 (and partial β1) stimulation raises resting heart rate by 20-30 bpm; sustained tachycardia stresses the myocardium.
- Muscle tremor, especially of the hands: a classic dose-dependent effect of peripheral β2-receptors, appearing even at a 20 μg starting dose.
- Muscle cramps and hypokalemia: β2 stimulation shifts potassium intracellularly; low serum potassium causes cramps and, rarely, rhabdomyolysis (taurine and magnesium ease it).
- Insomnia, anxiety and restlessness: the 35-40 hour half-life prolongs the stimulant effect, and evening dosing worsens sleep.
- Left ventricular hypertrophy (LVH): chronic use over 8 weeks causes documented LVH, fibrosis and arrhythmia in animal studies, with human cardiomyopathy cases reported.
- Excessive sweating, hot flushes and raised blood pressure: from the thermogenic β2 effect and sympathetic activation, worsened by dehydration.
- Headache and nausea: common sympathomimetic side effects, mainly during the upward phase of dose titration.
- Risk of acute overdose due to the long half-life: tachyarrhythmia, chest pain, vomiting and hypokalemia with toxicity symptoms (poisons-centre cases documented).
Contraindications · 7
- Any cardiovascular disease: hypertension, arrhythmia, ischaemic heart disease, prior heart attack or a positive family history are absolute contraindications.
- Cardiomyopathy and left ventricular hypertrophy: clenbuterol further worsens myocardial hypertrophy and fibrosis, raising the risk of sudden cardiac death.
- Hyperthyroidism: an overactive thyroid combined with β2 stimulation causes extreme sympathetic overactivation and cardiotoxic load.
- Beta-blocker therapy: beta-blockers pharmacologically conflict with clenbuterol, and the combination can cause unpredictable cardiovascular reactions.
- Pregnancy and breastfeeding: teratogenic and cardio-developmental effects in animal studies, and contraindicated due to its uterine-relaxant effect.
- Epilepsy or a history of seizures: β2 stimulation and the accompanying hypokalemia lower the seizure threshold.
- Severe anxiety disorder or panic disorder: the stimulant, sympathomimetic profile worsens anxiety, palpitations and insomnia.
Related Performance Compounds
Same therapeutic category
Studies
Related research and clinical findings
Clenbuterol induces lean mass and muscle protein accretion, but attenuates cardiorespiratory fitness and desensitizes muscle β2-adrenoceptors in humans.
Hostrup M, Moesgaard L, Fischer M
Clenbuterol exerts antidiabetic activity through metabolic reprogramming of skeletal muscle cells.
Meister J, Bone DBJ, Knudsen JR
Clenbuterol toxicity: a NSW poisons information centre experience.
Brett J, Dawson AH, Brown JA
Rhabdomyolysis Secondary to Clenbuterol Use and Exercise.
Grimmer NM, Gimbar RP, Bursua A, Patel M
A Comparison of the Molecular Pharmacological Properties of Current Short, Long, and Ultra-Long-Acting β(2)-Agonists Used to Treat Asthma and COPD.
Proudman RGW, Baker JG
β2-adrenergic receptor agonists as neuroprotective agents in Parkinson disease: a review
Mittal S, Bjørnevik K, Im DS et al.
Telegram
Have a question about Clenbuterol?
Reach out to an advisor on Telegram. Performance compounds are presented with a harm-reduction approach, based on peer-reviewed evidence.
The information here is for educational and scientific purposes only. Performance-enhancing compounds (AAS, prohormones, stimulants, doping agents) are illegal without prescription in Hungary and most of the EU, and carry serious health and legal risks. WADA bans them in competitive sport. This is NOT a usage guide, and we do not encourage any illegal use. If you do use them, medical supervision and regular bloodwork are ESSENTIAL. Severe endocrine, cardiovascular, hepatic and psychiatric side effects are possible.