Hmm, well, for one, ephedrine stimulates the beta adrenergic receptors which in turn will increase activation of the sympathetic nervous system, resulting in an increase in catecholamines, thyroid production etc. However, with continual exposure, these receptors will down regulade, thus minimizing the stimulating effect experienced from increases in sympathetic activity.
However, some studies have shown that even though the stimulating effect minimizes, the fat burning effects are still occuring. I would say keep going with it. | 
