Clenbuterol works the same way as ephedrine. However, unlike ephedrine, which is out of the body in a few hours, clenbuterol lasts for days. So you are on it 24 hours a day. No one chooses to be on ephedrine 24 hours per day, because it interferes with sleep, and so clenbuterol is more effective.

Also, for the same effect on fat cells, clenbuterol accelerates heart rate less, so one can use effectively a higher dose. (Not a greater quantity, but a dose giving a greater effect on fat cells for the same effect on tachycardia.) It results in severe downregulation of beta receptors, which moderate ephedrine use does not do.

Thus, it is particularly effective only for a short time. Typically if using clenbuterol the dose would be about 100 mcg per day. It is not anabolic (or significantly so.)