Clenbuterol
Also known as: clen
A bronchodilator misused for fat loss, with a modest effect and a genuine cardiac risk that scales with it.
Legal status: UK: prescription-only medicine; legitimately available only via a doctor's prescription. Checked 2026-07-26.
Prohibited in sport: Yes — on the WADA Prohibited List.
Approved medical use: Licensed in some countries as a bronchodilator for asthma; not licensed for human use in the UK. Also used veterinarily.
What it is
Clenbuterol is a long-acting beta-2 agonist, chemically related to the salbutamol used in asthma inhalers but far more persistent in the body. It is not licensed for human use in the UK. It raises metabolic rate and core temperature and promotes the release of fat from storage.
Where it is licensed abroad it is a prescribed asthma treatment at levels far below those used for fat loss, and it has veterinary uses in some countries: livestock use has caused mass food-poisoning incidents where it was used illegally.
What’s claimed
- Significant fat loss without dieting
- Muscle preservation or even muscle gain
- Improved breathing and work capacity in training
What the evidence shows
It does raise metabolic rate and fat oxidation in humans, but modestly, and the effect diminishes as the body's receptors downregulate within weeks (Bronze). The muscle-sparing claim comes from livestock studies where the animals were given far higher relative exposures than any human would tolerate; it has not been shown in people. Meanwhile the cardiac effects do not diminish, which is the central problem: tolerance builds to the benefit and not to the risk.
The risks
- Cardiac: tachycardia, palpitations, arrhythmia and, in case reports, myocardial ischaemia and infarction; clenbuterol poisoning is a recognised emergency-department presentation.
- Neuromuscular: pronounced tremor, muscle cramps and headache, driven partly by potassium shifts.
- Metabolic: falls in blood potassium and phosphate, which contribute to the arrhythmia risk.
- Anxiety, insomnia and sweating are near-universal at the levels used for fat loss.
- Product risk: manufactured outside any regulatory system. Independent testing repeatedly finds these products mislabelled, under- or over-concentrated, or containing entirely different active drugs, so the real exposure is unknowable from the label.
What needs monitoring
- Heart rate, rhythm and blood pressure
- Blood potassium and other electrolytes
- Any chest pain, fainting or sustained palpitation is an emergency, not a side effect to push through.
Sources
- Clenbuterol abuse in bodybuilding and athletics: unsupervised use, psychological motivations and health consequences (review).
- World Anti-Doping Agency. The Prohibited List.
- Pope HG, et al. Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement. Endocr Rev. 2014;35(3):341-375.