What special precautions should I follow? Sildenafil usually should not be taken more than once every 24 hours. It’s best to check your chosen provider’s site for more information. After discussing your erectile dysfunction and your medical history, ask your doctor for a prescription for Viagra. Taking generic ED medications is an effective way to save money without sacrificing quality. These PDE5 inhibitors may cause headache, flushing, drop in blood pressure, or other adverse effects.The generic name for Viagra is sildenafil. The drug also has another brand name: Revatio. It works by slowing down the action of PDE5, an enzyme in the blood vessel walls.. This enzyme breaks down. Dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may also occur. If. VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Patients should always ask their doctors for medical advice about adverse events. You are encouraged to report adverse events related to Viatris products by calling (U.S. only). If you prefer, you may contact the U.S. Food and Drug Administration (FDA.