Severe liver disease. Popular ED medications found online include: Viagra sildenafil : As the first FDA-approved PDE5 inhibitor for ED, sildenafil is taken 30 minutes to four hours before sexual intercourse to help achieve and maintain an erection. 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. These medicines are called PDE5 inhibitors. Avanafil Stendra. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. Never purchase ED medications from a site or service that doesn’t require a prescription and medical documentation.Walmart $ retail Save 99% $ One-time offer Get free savings Walgreens $1, retail Save 99%. Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. For ED treatment, PlushCare physicians can prescribe brand-name or generic Viagra, Cialis.