In addition, patients were asked a global efficacy question and an optional partner questionnaire was administered. Counterfeit medicines can be very dangerous. The most common side effects of VIAGRA: headache; flushing; upset stomach; abnormal vision, such as changes in color vision such as having a blue color tinge and blurred vision; stuffy or runny nose; back pain; muscle pain; nausea; dizziness; rash. If your dose is different, do not change it unless your doctor tells you to do so. Especially tell your healthcare provider if you take any of the following: medicines called nitrates medicines called guanylate cyclase stimulators such as Adempas riociguat medicines called alpha-blockers, such as Hytrin terazosin HCl , Flomax tamsulosin HCl , Cardura doxazosin mesylate , Minipress prazosin HCl , Uroxatral alfuzosin HCl , Jalyn dutasteride and tamsulosin HCl , or Rapaflo silodosin. A starting oral dose of 25 mg should be considered in those patients [see Dosage and Administration 2. Use only the brand of this medicine that your doctor prescribed.VIAGRA can cause your blood pressure to drop suddenly to an unsafe level if taken with certain other medicines. Do not take VIAGRA if you take any other medicines called “nitrates.” Nitrates are used to treat chest pain (angina). A sudden drop in blood pressure can cause you to feel dizzy, faint, or have a heart attack or stroke. Compare prescription drug prices and find coupons at more than 70, US pharmacies. Save up to 80% instantly! Who may not be able to take sildenafil. Sildenafil is not suitable for some people. To make sure it’s safe for you, speak to your doctor or pharmacist before taking sildenafil if you: have ever had an allergic reaction to sildenafil or any other medicine. have recently had a stroke, heart attack or a heart problem – your doctor should.