Its effects often last for 4 to 5 hours. This is important for your overall health and safety, as certain medications can negatively interact with each other or cause existing conditions to worsen. Medicines that you take by mouth are called oral medicines. People who use nitrates in any form or are being treated for pulmonary hypertension should not take sildenafil. It’s available in 10 mg and 20 mg tablets.Viagra (sildenafil) starts working in 30–60 minutes: depends on insurance, location, and pharmacy: 4 hours: different doses available available in generic form at a lower price: can interact. 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 1 . VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Not actual size. TIPS FOR TAKING VIAGRA. 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). Stop taking VIAGRA and call your healthcare provider right away if you have any.