Clenbuterol

Key facts

• Clenbuterol is a long-acting beta-2 agonist. Where it is approved, it is prescribed as a bronchodilator for asthma and other airway narrowing; it is not licensed for human use everywhere.
• It stays active in the body for over a day, so repeated dosing builds up rather than clearing between doses.
• Because it stimulates the heart as well as the airways, it can cause a fast or irregular heartbeat even at recommended doses, and overdose (including doses taken for bodybuilding or weight loss) has caused serious heart injury and death.
• Seek urgent care for chest pain, a racing or irregular heartbeat, or severe tremor with confusion.

What clenbuterol treats

Where prescribed, clenbuterol relieves bronchospasm, the tightening of airway muscles seen in asthma and other obstructive lung conditions. It has no genuine role in weight loss or muscle building. That non-medical use is widespread but is not a treatment indication, and it is a substance banned in competitive sport.

How clenbuterol works

Clenbuterol stimulates beta-2 receptors, which relax the smooth muscle around the airways so they widen. The same receptors sit in heart tissue and skeletal muscle, which is why the drug also raises heart rate, causes tremor, and shifts how the body burns fuel.

Before you take it

• Avoid it if you have a heart rhythm disorder, uncontrolled high blood pressure, or an overactive thyroid.
• Tell your prescriber about diabetes, since it can raise blood sugar, and about pregnancy or breastfeeding.
• Do not combine it with other stimulants, decongestants, other beta-agonists, or MAOI antidepressants; the combined effect on heart rate and blood pressure can be dangerous.

Side effects

Common effects include hand tremor, a fast heartbeat, muscle cramps, headache, and feeling jittery.

Stop and seek urgent medical care for any of these:

• Chest pain or pressure.
• A racing, pounding or irregular heartbeat.
• Severe shortness of breath.
• Sudden swelling of the face, lips or tongue.

Safety essentials

• Never exceed the prescribed dose. Because clenbuterol accumulates in the body, taking extra to compensate for a slow effect raises the risk of a dangerous fast or irregular heartbeat.
• Do not combine it with other stimulants or decongestants, since the added strain on the heart can be serious.
• Do not use clenbuterol for weight loss or bodybuilding. This unsupervised use has led to fatal heart events, and products sold for that purpose are frequently mislabelled or counterfeit.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.