Clenbuterol — Beta-2 Adrenergic Agonist — Weight & Metabolism
Selective beta-2 agonist with thermogenic and anti-catabolic properties. Approved for asthma in some countries. Not FDA-approved for human use in the US.
Overview
Clenbuterol is a selective beta-2 adrenergic receptor agonist originally developed for the treatment of asthma and other respiratory conditions. It is approved for asthma in some European and Asian countries but is NOT FDA-approved for human use in the United States, where it is approved only for veterinary use in horses (Ventipulmin). Clenbuterol's popularity in bodybuilding stems from its potent thermogenic (fat-burning) and anti-catabolic (muscle-preserving) properties. By activating beta-2 receptors in adipose tissue, clenbuterol stimulates lipolysis through the cAMP/PKA cascade, increasing free fatty acid mobilization and basal metabolic rate. In skeletal muscle, beta-2 agonism activates the Akt/mTOR pathway, promoting protein synthesis and inhibiting proteasomal degradation — producing an anti-catabolic effect that preserves lean mass during caloric deficit. Clenbuterol has a long half-life (approximately 26-48 hours), causing sustained sympathetic stimulation. Common side effects include tremor, tachycardia, palpitations, insomnia, muscle cramps (potassium depletion), and hypokalemia. Beta-2 receptor downregulation occurs rapidly (within 2 weeks), necessitating cycling. Serious cardiovascular toxicity has been reported, including cardiac hypertrophy, arrhythmias, and even death at high doses or with prolonged use.
Indications
- Approved (some countries): Asthma and COPD (NOT approved in US)
- Veterinary (US): Airway obstruction in horses
- Unapproved: Fat loss and body composition (bodybuilding use)
- Unapproved: Anti-catabolic agent during caloric restriction
Mechanism of Action
Selectively activates beta-2 adrenergic receptors in adipose tissue and skeletal muscle, stimulating intracellular cAMP production via adenylyl cyclase
Dosing
| Compound | Dose | Frequency | Notes |
|---|---|---|---|
| Clenbuterol | 20-40 mcg | Once daily (morning) | Starting dose — titrate up slowly |
| Clenbuterol | 80-120 mcg | Once daily | Common bodybuilding dose — significant side effects at these levels |
Evidence Grade
GRADE C
Safety & Contraindications
- Tachycardia, palpitations, and cardiac arrhythmias
- Cardiac hypertrophy with chronic use — potentially irreversible
- Tremor (hands especially) — dose-dependent, common
- Insomnia due to long half-life (26-48 hours)
- Hypokalemia and muscle cramps (potassium supplementation often needed)
- Headache, sweating, and anxiety
- Beta-2 receptor downregulation within 2 weeks — cycling required
- Not FDA-approved for human use in US
- WADA prohibited substance
- Clenbuterol-contaminated meat has caused positive drug tests