From
Medscape: Medscape Access
Medscape: What toxic effects are seen in horses?
Dr. McKeever: Therapeutic doses of clenbuterol given for a moderate length of time -- something well within the scope of what's in the brochure that accompanies the medication -- can not only push a horse towards heart failure, but it also results in other problems. It leads to a decrease in aerobic capacity, a decrease in the time to fatigue, and a shift of muscle fiber profile. When you look at the myocin heavy chain profile on muscle biopsies, you can see a change in the distribution of the different types of muscle fiber. We did research using standard-bred racehorses, which are middle distance runners. Normally, their profiles have a lot of fast-twitch oxidative fibers ('weight lifter' fiber), slow-twitch oxidative fibers (endurance fibers), and a moderate amount of the fast-twitch glycolytic fibers (which have a lot of oxidative capacity). When the horses were on clenbuterol, they shifted more towards a sprinter or weight lifter's profile - where the fast-twitch glycolytic fibers and the fast-twitch oxidative fibers predominate.
In other words, the tissue shifts from looking like it's from an aerobic athlete's muscle profile (as far as the fiber typing), to a profile that looks like it's from a weight lifter. That's over the course of 8 weeks of treatment. As for repartitioning effects, we saw those occurring within 2 weeks. As far as acute effects in the horses -- ie, the nervousness, sweating, elevated pulse rate, increases in blood pressure, muscle tremors -- we saw those as we gave the initial doses and when we wrapped up the dose. It's a very potent drug. The acute effects can be scary, but then the long-term effects look even scarier.
Medscape: Body builders who abuse this drug seem to use a 2 weeks on 2 weeks off regimen. Why? Would they get into medical trouble if they didn't cycle like this?
Dr. Hoffman: Probably not. Therapeutically, people such as asthmatics use it long term. In general, bodybuilders tend to cycle the drugs they use and that's probably based on their patterns of steroid use. If somebody stays on steroids too long they get the adverse effects. Usually bodybuilders will cycle steroids for 4 weeks - on for a month and off for a month, or on for 6 weeks off for 6 weeks. It's a general principle they apply to clenbuterol as well. Clenbuterol is not dangerous when used long term (at therapeutic doses) but people definitely develop a type of tolerance to, or tachyphylaxis. Initially when someone starts taking clenbuterol just to take 40 mcg of it, a 40 mcg dose they may barely tolerate it but after a week's use. If you look at the body building sites you'll see they tell people to up the dose. Start with a dose of 50 mcg or thereabouts, and you up it until you can tolerate 100 mcg. After a certain amount of time it is much easier to tolerate that dose; your body has developed tachyphylaxis. For that reason people will usually cycle clenbuterol. Two weeks is a shorter cycle. More people take clenbuterol for a month or even 6 weeks.