This is consistent with ritonavir’s marked effects on a broad range of P substrates. 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. The patients in studies had varying degrees of ED. National Library of Medicine. For erectile dysfunction, it’s best to take sildenafil on an empty stomach. You are encouraged to report adverse events related to Viatris products by calling U. The usual adult dose is 20mg taken 3 times a day.Viagra (sildenafil) and cost. Viagra (sildenafil) is a brand-name oral tablet prescribed for erectile dysfunction. The cost of the drug with and without insurance can depend on several factors. For most people, the maximum recommended dosage of Viagra is mg once per day. This dosage may be too high for people who have certain medical conditions or take . Sildenafil citrate is a white to off-white crystalline powder with a solubility of mg/mL in water and a molecular weight of VIAGRA is formulated as blue, film-coated rounded-diamond-shaped tablets equivalent to 25 mg, 50 mg and mg of sildenafil for oral administration.