Clenbuterol is a drug known as a beta-two agonist, and it is most commonly used to treat asthma by relaxing the muscles surrounding the trachea (passage from the mouth to the lungs) so that the patient may absorb greater amounts of oxygen. Beta-two agonists also affect the heart and muscles of the body. For this reason beta-two agonists must be used with caution, especially in patients with heart disease, diabetes, high blood pressure, or seizure disorders.


Clenbuterol has been shown to decrease the breakdown of muscle tissue and to stimulate the synthesis of muscle protein. These two processes result in an increase in muscle mass of both slow-twitch (endurance) and fast-twitch (strength and sprinting) muscle fibers. Clenbuterol also increases the speed of muscle contractions and promotes lipolysis (the breakdown of fat).


  • Increased fatigue.

  • Nausea.

  • Weakness.

  • Dizziness.

  • Muscle cramps.

  • Flushing.

  • Restlessness.

  • Irritability.

  • Tremor.

  • Headache.

  • Feeling your heart beat rapidly in your chest (palpitations).

  • Difficulty getting to sleep (insomnia).

  • Difficulty with urination.

  • Vomiting.

  • Vertigo (feeling as though the room is spinning).

  • Elevated blood pressure.

  • Unusual taste in the mouth.

  • Dry mouth and throat.

  • Ringing in ears (tinnitus ).

  • Rapid heart rate (tachycardia).


Like all beta-two agonists, clenbuterol is absorbed into the body when taken orally. The time from ingestion to affect is approximately 30 minutes. For the treatment of asthma, prescriptions of clenbuteral typically instruct patients to take the medication twice daily. The muscle growth effect of the medication typically lasts for a short period and decreases with prolonged use. Clenbuterol is generally used after anabolic steroids have been stopped.


Athletes should not use cenbuterol as a method of enhancing athletic performance. Few studies have been conducted on the drug, thus little is known of the possible side effects. Many athletic events routinely test for the use of clenbuterol.