Ephedrine is a naturally occuring central nervous system stimulant obtained
from the plant Ephedra equisetina. It is also produced by chemical
synthesis, the synthetic product being marketed in the form of its salt,
ephedrine sulfate; it occurs as a white crystalline powder with a bitter
taste, soluble in water and very soluble in alcohol. Ephedrine is closely
related in structure to methamphetamine, although its CNS actions are much
less potent and also longer-acting than those of the amphetamines. Its
peripheral stimulant actions are similar to but less powerful than those of
epinephrine (also called adrenaline).

Ephedrine is an adrenergic drug that works by stimulating alpha
and beta receptors thus causing the release of norepinephrine.
Alpha and beta receptors exist in the sympathetic nervous system,
(fight or flight) and stimulation causes increased heart rate,
bronchodilation, and vasoconstriction.

(from Drugs and Drug Abuse, 2nd Ed., by: Cox, Jacobs, LeBlanc, Marshman, and Fehr, 1987)

If you're having a problem with tolerance, make sure you take plenty of L-Tyrosine which will help build up your
epinephrine stores. Failing that, abstain for a week or so.