Clenbuterol was developed in the 1960s and later marketed by Boehringer Ingelheim. In several European countries, Latin America and parts of Asia it has been sold under names such as Spiropent, and in combination with the mucus thinner ambroxol (for example, Spasmo-Mucosolvan). For horses, it is sold as Ventipulmin. In the United States, the FDA has approved it only for horses that are not used for food.
Clenbuterol belongs to a class of drugs called beta-2 adrenergic agonists, the same broad family as the common asthma inhaler salbutamol (albuterol). It is not a steroid and not a hormone, even though it is often sold alongside steroids.
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Property |
Detail |
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Generic name |
Clenbuterol (usually as clenbuterol hydrochloride) |
|
Brand names |
Spiropent, Ventipulmin (veterinary), Spasmo-Mucosolvan (with ambroxol), others |
|
Drug class |
Long-acting beta-2 adrenergic agonist (bronchodilator) |
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Chemical name |
4-amino-α-\[(tert-butylamino)methyl\]-3,5-dichlorobenzyl alcohol |
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Molecular formula |
C12H18Cl2N2O |
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Molecular weight |
about 277.2 g/mol (base); about 313.7 g/mol (hydrochloride) |
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Forms |
Tablets (typically 20 micrograms), syrup, veterinary gel and granules |
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Half-life |
About 25 to 39 hours |
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Prescription status |
Prescription only where approved for humans; veterinary only in the US |
The doses involved are tiny. Medical clenbuterol is measured in micrograms (mcg), millionths of a gram, not milligrams. A typical human tablet contains 20 mcg. This matters, because confusing micrograms and milligrams, or using unregulated liquids, has caused serious poisonings.
Two chlorine atoms on its ring structure make clenbuterol resistant to breakdown, which is why it lasts much longer in the body than salbutamol.
The body has several types of adrenaline (adrenergic) receptors. Beta-2 receptors are found mainly in the smooth muscle of the airways, but also in blood vessels, skeletal muscle, fat tissue, the liver and, to a smaller degree, the heart. Clenbuterol switches these receptors on, much like a long-lasting dose of adrenaline aimed mostly at beta-2 sites.
With continuous use, beta-2 receptors become less sensitive and fewer in number, a process called downregulation. Within a couple of weeks, the fat-burning and bronchodilator effects weaken, while many side effects remain. This is one reason medical guidelines prefer other asthma treatments for long-term control.
Although clenbuterol favors beta-2 receptors, it is not perfectly selective. At higher doses it also stimulates beta-1 receptors in the heart and causes blood vessels to widen, which makes the heart beat faster to compensate. Animal studies show that repeated high doses can thicken the heart muscle and damage heart cells.
With a half-life of more than a day, clenbuterol builds up over several days of use. Even very small amounts can be found in urine for days to weeks, which is why athletes have tested positive after eating contaminated meat in some countries.
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Use |
Status |
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Asthma and chronic obstructive airway disease in humans |
Approved in some countries (not the US); used less today as better inhalers exist |
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Bronchitis with thick mucus (with ambroxol) |
Approved combination products in some countries |
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Airway obstruction in horses (recurrent airway obstruction, "heaves") |
Approved veterinary use, including in the US for non-food horses |
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Weight loss or "cutting" in humans |
Not approved anywhere; unsafe misuse |
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Muscle growth in humans |
Not approved; evidence is weak |
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Growth promotion in cattle, pigs or poultry |
Banned in the US, EU, China and many other countries |
In modern asthma care, inhaled treatments, such as inhaled corticosteroids combined with a long-acting inhaled bronchodilator, are preferred because they work directly in the lungs at far lower doses and with fewer whole-body side effects.
Where it is licensed for people, clenbuterol is taken by mouth as a tablet or syrup, usually twice a day. The figures below reflect standard prescribing information for adult asthma in those countries and are shown only to explain medical use. Your doctor sets the dose.
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Detail |
Standard prescribing information (adult asthma, where licensed) |
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Usual dose |
20 micrograms (0.02 mg) twice a day |
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Form |
20 mcg tablet, or measured syrup |
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Timing |
Morning and evening, at regular intervals |
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Children |
Weight-based syrup doses only where licensed, under a doctor |
There is no approved dose for weight loss or body composition. The amounts and "cycles" promoted online are not medically supported and are far above the asthma dose, which is why serious side effects are so common in people who misuse it.
Veterinary clenbuterol (for example, horse syrup or gel) is dosed for large animals and must never be used by people. Poison centers have reported many cases of people becoming seriously ill after using veterinary or online liquid products.
Tell your doctor about every health condition and medicine you have before taking clenbuterol. The biggest concerns are the heart, potassium levels and blood sugar.
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Medicine |
Possible effect |
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Beta-blockers (e.g. propranolol, atenolol) |
Block clenbuterol's effect and can trigger bronchospasm in asthma |
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Other stimulants (caffeine, ephedrine, decongestants, amphetamines, yohimbine) |
Much higher risk of fast heart rate, high blood pressure and arrhythmia |
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Diuretics, corticosteroids, theophylline |
Further lower potassium |
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Digoxin |
Low potassium raises the risk of digoxin toxicity |
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MAO inhibitors and tricyclic antidepressants |
Stronger effects on heart and blood vessels |
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Thyroid hormone |
Added stimulant effects on the heart |
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Drugs that prolong the QT interval |
Higher risk of dangerous heart rhythms |
Clenbuterol is listed by the World Anti-Doping Agency (WADA) as an anabolic agent, banned at all times, in and out of competition. There is no minimum threshold, so even tiny amounts can lead to a positive test. Athletes travelling to countries where it has been found in meat are often warned about food sources.
Even at medical doses, clenbuterol commonly causes stimulant-type side effects. At the much higher amounts used for weight loss, these become far more frequent and can be life-threatening.
Get emergency medical help (call your local emergency number) for:
Reports from poison centers and hospitals describe people misusing clenbuterol who developed abnormal heart rhythms, heart attacks, heart muscle damage, very low potassium and, in some cases, died. Some of these were young, otherwise healthy adults.
Where clenbuterol has been illegally fed to livestock, people have become ill after eating contaminated meat or liver, with tremor, racing heart, headache and dizziness. Outbreaks have been reported in Europe, China and Mexico. This is the main reason the drug is banned in animal feed.
Risks of misuse and fake products
Online clenbuterol is frequently sold as unregulated liquids or tablets with unknown strength. Errors between micrograms and milligrams can lead to overdoses many times the intended amount. It is also often combined with other stimulants or thyroid hormone, which greatly increases the risk to the heart. Any weight lost while taking it tends to return after stopping.
Check your local laws before buying or importing it. Products sold online "for research" are unregulated and not intended for human use.
Clenbuterol is a long-acting bronchodilator with a narrow medical role: treating asthma in a few countries and airway disease in horses. It works by switching on beta-2 receptors, which opens the airways but also speeds up the heart and metabolism. That stimulant effect is what drives its popularity as a fat burner, yet it is not approved for weight loss anywhere, its effects fade quickly, and its risks, including tremor, low potassium, dangerous heart rhythms and heart damage, are serious. If you are looking for help with weight or body composition, a doctor can guide you toward safer and proven options.
Medically, it treats asthma and airway obstruction in some countries and airway disease in horses. It is not approved anywhere for weight loss.
No. It is a beta-2 agonist bronchodilator, related to asthma drugs like salbutamol. It is often confused with steroids because it is sold and misused alongside them.
It depends on the country. In the US it is approved only for horses, and using it in people is not approved. In some countries it is a prescription asthma medicine.
It can slightly raise metabolism and fat breakdown for a short time, but the effect fades within weeks as the body adapts, and the risks to the heart are significant. It is not a safe or approved weight-loss method.
In livestock, high doses increase lean muscle. In humans, good evidence is lacking, and it is not approved for this.
Its half-life is more than a day, and it can be detected in urine for days to weeks, depending on the dose and test sensitivity.
It lowers blood potassium and can affect other electrolytes, which can lead to muscle cramps and weakness.
Yes. Heart attacks, dangerous heart rhythms and heart muscle damage have been reported, including in young, healthy people who misused it.
Yes. Athletes have tested positive after eating meat from animals illegally fed clenbuterol in some countries. WADA has no minimum threshold for it.
Both are beta-2 agonists. Salbutamol is short-acting and usually inhaled for quick relief. Clenbuterol is long-acting, taken by mouth, and has stronger whole-body effects.
If you have chest pain, a racing or irregular heartbeat, fainting or severe cramps, seek emergency care right away. For advice, contact your local poison control center.
No. These are dietary supplements with different ingredients, usually stimulants or herbs. Some have been found to contain undeclared drugs, so treat them with caution.