Sildenafil

Precautions
Do not exceed the recommended dose without checking with your doctor.
Do not take this medicine more than once a day.
Do not take this medicine if you take or use Nitroglycerin, including nitroglycerin tablets, patches, or ointment, or other Nitrates, such as isosorbide. If you are not sure whether a certain medicine is a nitrate, contact your doctor or pharmacist.
Use of this medicine will not prevent the spread of sexually transmitted diseases (STDs), including human immunodeficiency virus (HIV). Before you begin taking any new medicine, either prescription or over-the-counter, check with your doctor or pharmacist.

Drug Interactions
Tell your doctor of all nonprescription and prescription medication you may use, especially any nitrate medications (e.g., nitroglycerin, isosorbide dinitrate), nitroprusside (any "nitric oxide donor" medicines), cimetidine, erythromycin, azole antifungals (e.g., itraconazole, ketoconazole), mibefradil, rifamycins (e.g., rifampin), high blood pressure medicines, any other drug used to treat impotence, such as alprostadil (Caverject, Muse, Edex) or yohimbine (Yocon, Yodoxin, others)