Please expect up to 4 to 6 weeks for reimbursement. In vivo studies: No significant interactions were shown with tolbutamide mg or warfarin 40 mg , both of which are metabolized by CYP2C9. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Nitric oxide is a substance that causes blood vessels to relax. Your doctor will tell you not to take sildenafil if you are taking street drugs containing nitrates. Tell your doctor if any of these symptoms are severe or do not go away: headache. If you are taking sildenafil for erectile dysfunction, your doctor will probably start you on an average dose of sildenafil and increase or decrease your dose depending on your response to the medication.Headache, flushing, and upset stomach are common Viagra side effects. These effects are usually mild and often resolve on their own. While rare, Viagra can cause an erection that lasts longer than 4 hours (priapism). This can result in permanent damage if left untreated. Vision changes are also possible. Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the . Used for Erectile Dysfunction. Sildenafil (Viagra) is taken by mouth to treat erectile dysfunction (ED). You take it only as needed, typically about an hour before sexual activity (but you can take it anywhere between 30 minutes to 4 hours before sex). Don’t take sildenafil (Viagra) more than once a day because doing so can put you at higher.