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Clenbuterol is a popular - but banned - drug used by athletes in body-building, power-related, and even endurance sports. The chemical is attractive to athletes because it appears to have an anabolic effect on human muscles, and it may also increase fat metabolism. However, the actual effects of long-term clenbuterol intake on performance, muscle power, and overall health have been unclear.
In tests, sedentary rats which ingested clenbuterol had larger muscles than clenbuterol-free, sedentary rodents, and clenbuterol users also transformed leg-muscle cells from slow-twitch to fast-twitch fibres, a surprising change which would tend to increase anaerobic energy production and magnify muscle power during short, intense exertions.
However, clenbuterol also yielded three very negative changes. First, after just four weeks, clenbuterol-treated rats were unable to maintain their normal swimming or running training intensities, while clean rodents were quite capable of continuing. Secondly, the hearts of the clenbuterol-taking, trained rats increased dramatically in size compared to the hearts of sedentary rats
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