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. It may be difficult to remain firm despite wanting to have sex. Make sure to get an estimate of when you can expect your prescription in the mail. Finding an online ED provider can feel overwhelming. The mechanism of action for ED pills is simple: They inhibit the PDE5 enzyme which regulates the signals responsible for the smooth muscle tissue.Read on to learn where to buy Viagra and sildenafil online — plus, more about how Viagra works, its alternatives, and how it compares with other ED medications. Amazon Pharmacy: Sildenafil (Generic for Viagra, Oral Tablet).us. Account Returns& Orders. Cart. Transfer your prescription for Sildenafil (Generic for Viagra, Oral Tablet) on Amazon Pharmacy. Extra 15% off $30 sitewide with code SAVE15; Clip your mystery deal! BOGO 50% off select Walgreens health & wellness.