Clenbuterol is a drug that is not approved for human use in the U.S. In some countries, it is available by prescription only for people with asthma or other breathing problems. Since 1998, the FDA has allowed clenbuterol for treating horses with asthma. It is not allowed for animals that are used in food production.

Clenbuterol is a substance that has steroid-like effects and is classified as a beta2-adrenergic agonist. This means that it stimulates the beta2-adrenergic receptors in your throat. The medicine helps relax your muscles and lungs, making it easier to breathe if you have asthma or another respiratory condition. It can stay in your body for up to 39 hours after you take it.

However, clenbuterol — also called clen — is abused by athletes and bodybuilders for its ability to burn fat. The same receptors that are activated when taking clenbuterol for asthma also help to burn off fat and increase lean muscle mass. Athletes who use clenbuterol daily typically take 60 to 120 micrograms per day. Normally this is taken in combination with other performance-enhancing drugs or anabolic steroids.

Clenbuterol increases your body temperature through a process called thermogenesis. Once your body temp is up, your metabolism is then primed to burn off more calories. Since fat is stored in the body as energy, your body can use the calories that you already have stored. This can decrease your body fat and lower your overall weight.

Because is a bronchodilator, it opens up your airways when you take it. This is helpful for those with asthma. For athletes, this allows them to increase their stamina by having more airflow moving around the body. More oxygen is available, so you can perform harder and better.

Even though it isn’t legal in the US, athletes and bodybuilders continue to abuse clen to help them cut weight and increase muscle mass. Many see it as an alternative to anabolic steroids — the drugs that typically come to mind when you think of performance-enhancing substances. It has the reputation of being a “non-steroidal steroid” due to its ability to mimic steroids. Since it isn’t technically a steroid, some athletes saw clenbuterol for bodybuilding as a more “natural” approach to building muscle.

Even though it is illegal and has several side effects, many athletes still abuse clen. 

Fewer androgenic side effects. It’s thought that clenbuterol is more popular than anabolic steroids with female bodybuilders because there are fewer androgenic side effects. Steroids typically cause side effects like an increase in facial hair or the deepening of your voice. Clenbuterol is not known to cause these.

Rapid weight loss. As noted, clenbuterol works by raising your metabolism, helping you to burn off fat. One study involved two groups of higher weight men who were put on the same strict diet. One group was given clenbuterol and one wasn’t. Over ten weeks, the group who received clenbuterol lost an average of 11.4 kilograms of fat and the control group lost 8.7 kilograms of fat.

Appetite suppression. Many bodybuilders rely on clenbuterol before an upcoming performance or competition to trim off extra fat. A secondary effect of this drug is that it helps curb your appetite so that you take in fewer calories. Not every person experiences this effect, however.

Many athletes and bodybuilders use clenbuterol for its benefits — but there are several dangerous side effects to be aware of.

Some of the most common side effects include:

  • Heart palpitations
  • Tremors
  • Increased heart rate (tachycardia)
  • Lowered blood potassium (hypokalemia)
  • High blood sugar (hyperglycemia)
  • Anxiety
  • Agitation
  • Sweating
  • Cardiac arrest
  • Feeling hot or warm
  • Insomnia
  • Muscle cramps

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