It is well known that Clenbuterol use results in rapid down-regulation of beta 2 receptors. This is due to the powerful stimulatory effect of the drug. It is therefore pointless to use clen for long periods without a break. Some believe that a two day on, two day off dosing schedule will allow adequate potential for receptor up-regulation. However, I doubt this to be the case due to the relatively long half life of clen, resulting in continued stimulation even throughout the ‘off’ days. A much better regime would be a two week on, two week off cycle. Maximum plasma levels are reached around 2-3 hours after oral administration, and terminal half life at 34 hours (Zimmer, 1976).