Sudden vision loss in one or both eyes can be a sign of a serious eye problem called non-arteritic anterior ischemic optic neuropathy NAION. Its effects often last for 4 to 5 hours. This medicine also works best when you take it an hour before sex on an empty stomach. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. 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.Viagra is available in three strengths: 25 milligrams (mg), 50 mg, and mg. What are the typical dosages of Viagra? Typically, your doctor will start you on the recommended dosage. Then. If you’re holding out for a generic version of branded Viagra, you won’t have to wait much longer. Teva Pharmaceuticals plans to release its generic of the “little blue pill” as early as. Don’t take sildenafil (Viagra) more than once a day because doing so can put you at higher risk for serious side effects, such as vision loss or low blood pressure. It’s available as a lower-cost generic.