Clenbuterol

What it is

Like salbutamol, clenbuterol belongs to the beta-2 sympathomimetics, that is, substances that widen the bronchi. Unlike the well-known asthma inhalers, it acts for a very long time: the summary of product characteristics gives a half-life of 34 hours and a duration of action of up to 14 hours. In Germany it is approved as a tablet for the treatment of asthma and chronic obstructive bronchitis, prescription-only and only with a strict indication. In the US it is not approved for humans.

The second legitimate world is veterinary medicine. In the EU, beta agonists may be used only in horses against respiratory diseases and as an injection to inhibit contractions in cows at calving. As fattening agents in animals whose meat is eaten, they are prohibited.

How it works

Clenbuterol activates beta-2 receptors in the bronchi, blood vessels, heart, skeletal muscle and fat tissue. In muscle it increases protein synthesis via the messenger cAMP and protein kinase A. In livestock farming, this leads to more muscle and less fat deposition in cattle, pigs, poultry and sheep, hence its reputation as a repartitioning agent. In the heart, the same receptor family speeds up the pulse and increases the force of contraction, and the potassium level in the blood falls. Both explain the typical signs of poisoning.

Why people take it

In bodybuilding and fitness, clenbuterol is regarded as an agent for the cutting phase: lose fat, keep muscle. It is also used for weight loss outside of sport. At the poison information center in New South Wales, bodybuilding and weight loss were the most common reasons for taking it.

What the studies show

Randomized trial in healthy men

Hostrup and colleagues, Journal of Physiology 2025. 11 trained men, 2 weeks each of clenbuterol and placebo in a crossover. Result: 0.91 kg more lean mass, no effect on fat mass, 7 percent lower maximal oxygen uptake and 4 percent lower exercise capacity. Heart rate was 10 beats per minute higher 2 hours after intake. The signaling effect in the muscle was already waning within the 2 weeks. The authors consider the ban in elite sport justified.

Overweight and blood sugar

Van Lier and colleagues, Nature Communications 2026. 14 people with overweight or obesity aged 40 to 70, 4 weeks, double-blind against placebo. Insulin-dependent glucose uptake rose by 13 percent in the posterior thigh muscle and by 15 percent in the lateral one, without statistical certainty. Body weight, fat mass and lean mass did not change. Under clenbuterol, 3 participants reported hand tremor and 2 reported headache and palpitations.

Poisoning through meat

The outbreaks mostly originated from contaminated liver. In Catalonia in 1992, a total of 113 people fell ill after eating calf liver, more than half with nervousness, racing heart, muscle tremor and headache; there were no deaths. At the 2011 U-17 Football World Cup in Mexico, 109 of 208 urine samples contained clenbuterol, as did 14 of 47 meat samples from the team hotels; no one was suspended because contaminated meat was considered the cause.

Where the data stop

The core of the scene’s promise, fat loss, is not supported in humans. The redistribution of fat to muscle comes from livestock farming. The two randomized human trials found no change in fat mass, neither in lean trained men nor in people with overweight. Performance did not improve but worsened, and in heart failure, endurance fell under clenbuterol, while maximal strength increased by 27 percent compared with 14 percent under placebo. Both human trials are small and short, 2 and 4 weeks respectively. There are no controlled data on longer use or on the patterns circulating in the scene. The safety data on misuse come from poison center series and case reports, so it is not possible to quantify how frequent serious cardiac events are. A British study on heart failure with a ventricular assist device, in which 11 of 15 patients were able to have the device removed, tested a combination regimen of several drugs and says nothing about clenbuterol alone.

Status, approval and legal

In Germany, clenbuterol is a prescription-only medicine, approved since November 11, 1988 for asthma and chronic obstructive bronchitis. It is listed by name in the annex of the German Anti-Doping Act among the other anabolic agents. This means that trading and supplying it for doping purposes, as well as acquiring, possessing and importing it in non-small quantities for doping in sport, are prohibited; the threshold is set by the Doping Agents Quantity Ordinance of 2023. The penalty ranges up to 3 years, and in serious cases from 1 to 10 years. German customs explicitly names clenbuterol as an example. In EU animal husbandry it is prohibited as a fattening agent under Directive 96/22/EC. The World Anti-Doping Agency lists it in the 2026 list under S1.2 among the anabolic agents, prohibited at all times. Because meat can be contaminated in some countries, elite athletes risk an unintentional positive test; Mexico is the best-known example.

Safety

The most common complaints are racing heart, tremor, restlessness, headache, palpitations, muscle cramps and nausea. It becomes dangerous via the heart and metabolism. In a US case series involving clenbuterol-adulterated heroin, the median potassium was 2.5 mEq/L and lactate 9.4 mmol/L, cardiac markers were elevated, and all patients survived with treatment. A young bodybuilder without classic risk factors suffered a heart attack with a blood clot in the anterior coronary artery. The summary of product characteristics lists myocardial ischemia and low potassium as side effects and reports cases with fatal outcome after abusive overdose. In animal experiments, dose-dependent heart muscle necrosis occurred. Clenbuterol must not be used in severe hyperthyroidism, tachycardic arrhythmias and hypertrophic obstructive cardiomyopathy. Diuretics and other beta-2 agents increase potassium loss and the effect on the heart. Because of the long half-life, signs of poisoning subside only slowly. Children are affected too: a 13-year-old girl came to the emergency department with chest pain, tremor and a prolonged QT interval.

Sources

• Hostrup et al., J Physiol 2025 – randomized trial in healthy men: lean mass, fat mass, endurance
• Van Lier et al., Nat Commun 2026 – clenbuterol in overweight: glucose uptake, body composition
• Kamalakkannan et al., J Heart Lung Transplant 2008 – clenbuterol in chronic heart failure
• Brett et al., Med J Aust 2014 – New South Wales Poisons Information Centre, 63 exposures
• Hieger et al., J Emerg Med 2016 – case series of clenbuterol-adulterated heroin
• Salleras et al., Public Health Rep 1995 – outbreak in Catalonia, 113 cases
• Thevis et al., Drug Test Anal 2013 – clenbuterol at the U-17 World Cup in Mexico
• Aggarwal et al., BMJ Case Rep 2025 – heart attack in a young bodybuilder
• Mersmann, J Anim Sci 1998 – beta agonists and growth in livestock
• German Anti-Doping Act (Anti-Doping-Gesetz), § 2 to § 4 and annex