Prazosin has been administered without any adverse drug interaction in limited clinical experience to date with the following: cardiac glycosides: digitalis and digoxin; hypoglycemic agents: insulin, chlorpropamide, tolazamide and tolbutamide; tranquilizers and sedatives: chlordiazepoxide, diazepam and phenobarbital; agents for the treatment of gout: allopurinol, colchicine and probenecid; antiarrhythmic agents: procainamide and quinidine; and analgesic, antipyretic and anti-inflammatory agents: propoxyphene, ASA, indomethacin and phenylbutazone.
Addition of a diuretic or other antihypertensive agents to prazosin has been shown to cause an additive hypotensive effect (see Warnings and Dosage & Administration).
Drug/Laboratory Test Interactions: In a. study on 5 patients given from 12 to 24 mg of prazosin/day for 10 to 14 days, there was an average increase of 42% in the urinary metabolite of norepinephrine and an average increase in urinary VMA of 17%. Therefore, false positive results may occur in screening tests for pheochromocytoma in patients who are being treated with prazosin. If an elevated VMA is found, prazosin should be discontinued; and the patient retested after a month.