From the 1920's to the 1940's, ephedrine was used as a bronchodilator and decongestant. However its side effects of high blood pressure and heart palpitations led to its replacement by pseudo ephedrine which was equally effective but had fewer side effects. Pseudoephedrine is the active ingredient in many OTC products such as Sudafed.
Ma Huang's active ingredients include ephedrine, pseudoephedrine, and norpseudoephedrine. These are all strong central nervous system stimulants - stronger than caffeine but less potent than amphetamine. They will dilate lung airways, stimulate the heart, increase blood pressure, and increase the metabolic rate and perspiration. They also decrease the production of saliva and stomach acid.
Ma Huang is promoted as a weight loss product, especially when combined with caffeine. This effect may be the result of a slight increase in the metabolic rate as well as via a central appetite suppressing effect. There has been a suggestion that weight loss associated with ephedrine/caffeine products preserves muscle mass - a key marketing point to athletes.
However, the FDA has compiled more than 800 reports of adverse effects of Ma Huang including heart attacks, tremors, strokes, insomnia, and at least 17 deaths. It is not recommended for pregnant women (it can cause spontaneous abortions), people with cardiac problems, angina, hypertension, thyroid disease, or prostate enlargement. It can cause dangerous elevations of blood pressure in those on MAO inhibitors and alpha 2 blockers. As it is not regulated, the ephedrine content of commercial products is quite variable, another reason to avoid products containing this drug.
In addition to it's side effects, it is on the list of banned substances of the International Olympic Committee. For this reason, and also because of negative publicity, many manufacturers have responded by marketing "Ma Huang free" products.