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young guy smilingClenbuterol is sometimes referred to as a steroid by the media. By looking at the structure, you can clearly see that it is not a steroid. It is, in fact, a beta-adrenergic agonist very similar to epinephrine. Clenbuterol is used by athletes for several purposes. The primary purpose is as a stimulant and weight loss agent but secondarily it is also believed to be anabolic. Clenbuterol has been shown to be anabolic in animals and to cause muscle fiber type switching from slow to fast1,2,3. There is some evidence from animal studies that this anabolic response is at least partly mediated through increased muscle content of IGF-14. For this reason, it has been used as a feed additive to increase bulk of farm animals used for meat. Most people do not report anabolic effects from clenbuterol but this is likely due either to the much lower doses used in humans compared to animals or to comparison of results to those obtained from steroids. Clenbuterol does not cause bloating and will increase metabolic rate causing fat to be burned5. This results in a lower scale weight and since most people equate scale weight to gains, they do not consider clenbuterol to be anabolic. One problem with the use of clenbuterol is that beta-adrenergic receptors adjust quickly. For this reason, most cycle the use of clenbuterol in a two-week on, two-week off cycle. Some have advocated using clen on a 2 week on 2 week off cycle, but as the half-life of clen is rather long (about 36 hours) this cycle is probably not going to prevent receptor downgrade. Side effects are similar to other stimulants including rapid heartbeat, increased body temperature, nervousness, tremor, headache, perspiration, nausea and insomnia6,7. A particularly disturbing side effect is that clenbuterol has been shown in the scientific literature to cause enlargement of the heart8. Some have suggested that using clenbuterol between AAS cycles could help retain muscle mass by increasing androgen and thyroid receptor levels in skeletal muscles.


1. Dodd SL, Powers SK, Vrabas IS, Criswell D, Stetson S, Hussain R: Effects of clenbuterol on contractile and biochemical properties of skeletal muscle. Med Sci Sports Exerc. Jun;28(6):669-76, 1996

2. Emery PW, Rothwell NJ, Stock MJ, Winter PD: Chronic effects of beta 2-adrenergic agonists on body composition and protein synthesis in the rat. Biosci Rep. Jan;4(1):83-91, 1984

3. Maltin CA, Delday MI, Hay SM, Smith FG, Lobley GE, Reeds PJ: The effect of the anabolic agent, clenbuterol, on overloaded rat skeletal muscle. Biosci Rep. Feb;7(2):143-9, 1987

4. Awede BL, Thissen JP, Lebacq J: Role of IGF-I and IGFBPs in the changes of mass and phenotype induced in rat soleus muscle by clenbuterol. Am J Physiol Endocrinol Metab. Jan;282(1):E31-7, 2002

5. Rothwell NJ, Stock MJ: Influence of clenbuterol on energy balance, thermogenesis and body composition in lean and genetically obese Zucker rats. Int J Obes. 11(6):641-7, 1987

6. Hoffman RJ, Hoffman RS, Freyberg CL, Poppenga RH, Nelson LS: Clenbuterol ingestion causing prolonged tachycardia, hypokalemia, and hypophosphatemia with confirmation by quantitative levels. J Toxicol Clin Toxicol. 39(4):339-44, 2001

7. Petrou M, Wynne DG, Boheler KR, Yacoub MH: Clenbuterol induces hypertrophy of the latissimus dorsi muscle and heart in the rat with molecular and phenotypic changes. Circulation. Nov 1;92(9 Suppl):II483-9, 1995

8. Wong K, Boheler KR, Bishop J, Petrou M, Yacoub MH. Clenbuterol induces cardiac hypertrophy with normal functional, morphological and molecular features. Cardiovasc Res. 37(1):115-22, 1998