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What You Need to Know About Clenbuterol for Bodybuilding

Written by Donna Siegfried
Medically Reviewed by Jabeen Begum, MD on August 21, 2025
4 min read

In the U.S., clenbuterol is a prescription medicine used to treat blocked airways in horses. In other countries, people are prescribed clenbuterol to treat asthma or breathing problems. The FDA has not approved it for use by people in the U.S., and it's not allowed to be given to animals used in food production. Yet some athletes and bodybuilders use clenbuterol (or “clen”) to help them reach their fitness goals.

Here’s what you need to know about this powerful and risky medication.

 

‌Clenbuterol is a bronchodilator, which means it relaxes the muscles around the airways and opens them to make breathing easier. This is why it is prescribed to treat asthma and breathing problems.

It works by activating beta2-adrenergic receptors. Other similar bronchodilators are albuterol and formoterol, which are approved in the U.S. for asthma and other lung conditions.

Clenbuterol is a long-lasting bronchodilator, so it can stay in your body for up to 39 hours after you take it.

When clenbuterol is prescribed to treat asthma in other countries, the dose is usually 0.04-0.06 milligrams per day. Athletes and bodybuilders who use clenbuterol — also called clen — typically take two to three times that amount (0.06 to 0.12 milligrams) per day. Even at low doses, clenbuterol can be toxic.

There are several dangerous side effects to be aware of:

You are more likely to have side effects if you take higher doses of clenbuterol. As this drug stays in your body for quite a while, you could have side effects anywhere from one to eight days. Studies show that more than 80% of people who had serious side effects from clenbuterol had to be hospitalized.

If you take clenbuterol and experience any of these side effects, immediately stop using it and get help from a doctor.

Clenbuterol is not a steroid, but it acts a bit like one. It can build lean muscle mass and decrease fat. So, bodybuilders misuse clenbuterol to reshape their bodies. They may take clenbuterol along with anabolic steroids or growth hormone. Some may have body dysmorphia, thinking that their body is flawed and not muscular or strong enough.

Although not approved for use, it is banned by the World Anti-Doping Agency and the International Olympic Committee. It has several side effects, but many athletes still misuse “clen” for these reasons:

Burning fat

Clenbuterol increases the resting metabolic rate by 10%-50%. That means your metabolism is primed to burn more calories. 

Energy is stored in the body’s cells, including fat cells. When your body uses stored energy, it will burn the sugars and fats you’ve stored. This can reduce your body fat and overall weight.

Building muscle

The same beta2-adrenergic receptors that activate when taking clenbuterol for asthma also help increase lean muscle mass while decreasing body fat. Because of such effects, clenbuterol is sometimes seen as an alternative to anabolic steroids — the drugs that typically come to mind when you think of performance-enhancing substances. Anabolic steroids also increase lean muscle. 

“Clen” is not a steroid. But in animals, it has shown some effects similar to steroids. 

Increasing stamina

Because clenbuterol is a bronchodilator, it opens up your airways when you take it. Having more airflow increases the amount of oxygen available. For athletes, it allows them to perform harder and better. 

Clenbuterol is not approved for human use in the U.S. Outside of the U.S., some countries allow it to be prescribed for people with asthma or breathing problems. Some athletes and bodybuilders misuse clenbuterol to lose fat and gain muscle. They may take two to three times the amount prescribed to those with asthma. Serious side effects may occur, sometimes leading to hospitalization. Use of clenbuterol is prohibited by the World Anti-Doping Agency and the International Olympic Committee.

Does clenbuterol decrease testosterone? 

Anabolic steroids have been shown to lower testosterone levels and shrink the testicles in men. Although clenbuterol has steroid-like effects, such as building muscle, it is not technically an anabolic steroid. Animal studies show that clenbuterol may raise the production of testosterone. It’s unknown if clenbuterol has the same effect in humans. 

What class of drug is clenbuterol? 

Clenbuterol is a beta2-adrenergic agonist, also known as a bronchodilator. It is not FDA-approved in the U.S. for people. It is approved to treat blocked airways in horses but cannot be used in animals raised for food production.

What are the effects of clenbuterol for women?

Clenbuterol may be more popular than anabolic steroids with female bodybuilders because of fewer side effects. Anabolic steroids typically cause side effects such as increased facial hair, thickening of the skin, and deepening of your voice. Clenbuterol is not known to cause these.‌ Women taking anabolic steroids have reported disrupted menstrual cycles, but it's not known if clenbuterol affects menstruation.