This similarity protects cGMP from degradation because sildenafil can bind to the catalytic site to act as a competitive inhibitor of cGMP-specific PDE-5, the enzyme that normally catalyzes the breakdown of vasodilatory cGMP. Outline appropriate monitoring of sildenafil therapy in erectile dysfunction and pulmonary arterial hypertension. If you experience a prolonged or painful erection for 4 hours or more, contact your doctor immediately. The effects may be increased because of slower removal of the medicine from the body. If you have erection problems sildenafil works by temporarily increasing blood flow to your penis when you get sexually excited. The most common side effects are headaches, feeling sick, indigestion and dizziness. Can you get Sildenafil for women?Sildenafil tablets for pulmonary hypertension come in 10mg and 20mg strengths. The usual adult dose is 20mg taken 3 times a day. Doses for children can range from 10mg to 20mg taken 3 times a day, depending on their weight. Do not take it more than 3 times a day. Sildenafil is a prescription drug. It comes in the form of a tablet and suspension (liquid). Both are taken by mouth. It also comes in an intravenous (IV) form, which is only given by a doctor.  · The effects may be increased because of slower removal of the medicine from the body. Leukemia (blood related cancer) or. Multiple myeloma (blood related cancer) or. Sickle-cell anemia (blood disorder)—Sildenafil should be used with caution in these patients as problems with prolonged erection of the penis may occur.