Take sildenafil at around the same times every day. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive. 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. Make sure you tell your doctor if you have any other medical problems, especially: Abnormal penis, including a curved penis or birth defects of the penis—Chance of problems occurring may be increased and this medicine should be used with caution in these patients. Do not use more of it and do not use it more often than your doctor ordered. If too much is used, the chance of side effects is increased.Viagra contains the drug sildenafil. It belongs to a class of drugs called phosphodiesterase type 5 (PDE5) inhibitors. (A class of drugs is a group of medications that act in a similar way.). Generic name: sildenafil (oral) [ sil-DEN-a-fil ] Brand names: Revatio, Viagra Drug class: Impotence agents Medically reviewed by Sophia Entringer, PharmD. Last updated on Uses Warnings Before taking Dosage Side effects Interactions FAQ What is Viagra?  · For oral dosage form (tablets): Adults up to 65 years of age—50 milligrams (mg) as a single dose no more than once a day, 1 hour before sexual intercourse. Alternatively, the medicine may be taken 30 minutes to 4 hours before sexual intercourse. Your doctor may adjust your dose if needed.