Vardenafil usually works for 4 to 5 hours. Generic versions of sildenafil, vardenafil and tadalafil are available. Sildenafil Viagra. This medicine works best when you take it on an empty stomach an hour before sex. When oral medications might not be safe Check with your health care team before you take any medicine for erectile dysfunction. The most common side effects of VIAGRA: headache; flushing; upset stomach; abnormal vision, such as changes in color vision such as having a blue color tinge and blurred vision; stuffy or runny nose; back pain; muscle pain; nausea; dizziness; rash. This medication is taken with or without food an hour before sex.Sildenafil (Viagra, Revatio) is a moderately priced drug used to treat erection problems in test1.ru is also used to treat pulmonary arterial hypertension, a serious heart and lung . Viagra vs. over-the-counter products. Many over-the-counter (OTC) products may be advertised to help with ED. Examples of these products include: dehydroepiandrosterone . Viagra is usually taken only when needed, 30 minutes to 1 hour before sexual activity. You may take it up to 4 hours before sexual activity. Other drugs may interact with .