They’re often the first line of treatment for trouble getting or keeping an erection, called erectile dysfunction ED. Recent heart or blood vessel problems. Sildenafil for pulmonary hypertension can also be known by the brand names Revatio and Grandipam. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. Viagra generally only stays in your system for four to six hours. Continue to take sildenafil even if you feel well.Cost Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in. Generic Viagra (sildenafil) is available as a , , , and milligram (mg) pill and in liquid form. It’s meant to be taken as needed — about an hour before sex — but shouldn’t be. 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.