Sildenafil Citrate Synonyms: VIAGRA; uk92480-10 CAS#: 139755-83-2 MF: C28H38N6O11S MW: 666.7 Chemical Properties: White Solid Usage: An orally active selective type 5 cGMP phosphodiesterase inhibitor Dosage: As with all prescription drugs, proper dosage is at the discretion of a licensed medical doctor. The dose of sildenafil for erectile dysfunction is 25 mg to 100 mg taken not more than once per day between 30 minutes and 4 hours prior to sexual intercourse.