Captopril can interact with a number of drugs, including allopurinol, asprin, cyclosporine, digoxin, lithium, nitroglycerin and other heart medicines, other blood pressure medications, and diuretics. Because captopril can increase the potassium levels in the bloodstream, it may react with salt substitutes and other medications which are known to increase potassium levels. Avoid taking these medications while taking captopril. Also avoid pregnancy or breastfeeding. Always check with your doctor to see whether or not captopril will react with any other medications you might be taking.