How should this medicine be used? These are commonly prescribed for an enlarged prostate or male pattern baldness. This medicine also works best when you take it an hour before sex on an empty stomach. Take sildenafil as needed before sexual activity. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. This increased blood flow can cause an erection.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 . Sildenafil is in a class of medications called phosphodiesterase (PDE) inhibitors. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. . They’re often the first line of treatment for trouble getting or keeping an erection, called erectile dysfunction (ED). Oral medicines for ED usually work well and .