Sildenafil Viagra. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine. Tell your doctor if any of these symptoms are severe or do not go away: headache. Don’t ignore stress, anxiety or other mental health concerns. If you experience chest pain, dizziness, or nausea during sexual activity, call your doctor immediately and avoid sexual activity until your doctor tells you otherwise. If you have certain health conditions or are taking certain medications, your doctor may tell you to take sildenafil less often.FAQ What is sildenafil? Sildenafil relaxes muscles of the blood vessels and increases blood flow to particular areas of the body. Sildenafil under the name Viagra is used to . Why it’s used Sildenafil oral tablet is used to treat ED and PAH. Generic oral tablets are available for both conditions, but the brand-name drugs each treat only one of the . Take sildenafil as needed before sexual activity. The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 .