When hypertension occurs in the lungs, the heart must work harder to pump enough blood through the lungs. Take sildenafil at around the same times every day. The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St. What side effects can this medication cause? By controlling the enzyme, sildenafil helps to maintain an erection after the penis is stroked. What special dietary instructions should I follow? 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 minutes before sexual activity.Brand names: Viagra, Revatio. Sildenafil comes in three forms: an oral tablet, an oral suspension (liquid), and an injection that’s only given by a doctor. Sildenafil oral tablet is used to. Generic Viagra (sildenafil) is available as a , , , and milligram (mg) pill and in liquid form. It’s meant to be taken as needed — about an hour before sex — but shouldn’t be. Sildenafil is in a class of medications called phosphodiesterase (PDE) inhibitors. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. This increased blood flow can cause an erection. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily.