Finding an online ED provider can feel overwhelming. Medicines that block an enzyme called CYP3A4. What side effects can this medication cause? If you are taking sildenafil for PAH, take the missed dose as soon as you remember it. Some services offer a subscription option where they’ll send your medications every month at a discount. Sildenafil usually should not be taken more than once every 24 hours.By Ginger Skinner Last updated: June 14, If you’re looking for a less costly Viagra, you might want to ask your doctor about sildenafil. It’s the active ingredient in both . No in-person visit is required, and the first consultation is free. A brief review period: Your information will be reviewed by a U.S.-licensed physician within 24 to 48 hours to . Sildenafil (Viagra, Revatio) is a moderately priced drug used to treat erection problems in men. It is also used to treat pulmonary arterial hypertension, a serious heart and lung .