Chlorpromazine was found to reverse the antihypertensive effects of guanethidine six times in use with four moderately or severely hypertensive psychiatric patients. Haloperidol and thiothixene caused similar effects. It is postulated that this drug-drug interaction occurs because chlorpromazine blocks the neuronal amine-guanethidine uptake pump, thereby denying guanethidine access to its site of action.Abstract Teaser