If you prefer, you may contact the U. For treatment of pulmonary arterial hypertension: For oral dosage forms suspension or tablets : Adults—20 milligrams mg 3 times a day. 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. This medicine also works best when you take it an hour before sex on an empty stomach. This medicine works best when you take it on an empty stomach an hour before sex. Vardenafil usually works for 4 to 5 hours. These medicines all work well for ED.Viagra comes as tablets that you take by mouth, when needed. Available strengths: 25 mg, 50 mg, mg Viagra comes in these strengths: 25 milligram (mg), 50 . Common Viagra side effects may include: flushing (warmth, redness, or tingly feeling); headache, dizziness; abnormal vision (blurred vision, changes in color . Learn more about what VIAGRA® (sildenafil citrate), a prescription treatment for ED. 50 mg, and mg tablets. Patients should always ask their doctors for medical advice .