sildenafil
Meaning of sildenafil: Found by accident while a heart drug was being tested, it works only where nitric oxide is already being released.
Definition of sildenafil
Sildenafil is a phosphodiesterase type 5 inhibitor. By blocking the enzyme that breaks down cyclic GMP, it keeps a relaxing signal switched on in smooth muscle, so vessel walls loosen and blood flow rises wherever that signal is active. It treats erectile dysfunction and, at different doses under another brand name, pulmonary arterial hypertension.
How does it work?
Nerve endings release nitric oxide, which prompts cells in the wall of a blood vessel to make cyclic GMP. That messenger relaxes the surrounding smooth muscle and the vessel widens. Phosphodiesterase type 5 breaks cyclic GMP down and ends the effect, and sildenafil blocks that enzyme so the signal lasts longer and works harder. Nothing in the sequence starts without nitric oxide, which is why the drug does not produce an erection by itself and why sexual stimulation is still required. The compound was under investigation for chest pain from narrowed heart arteries when this effect was noticed in trial participants, and the observation redirected its development.
What is it used for?
| Use | How the effect helps |
|---|---|
| Erectile dysfunction | Arteries and erectile tissue relax so blood enters and is held |
| Pulmonary arterial hypertension | Lung arteries widen, easing the load on the right side of the heart |
The lung use requires a different dose and schedule and is sold under a separate name, which is why one molecule appears on prescriptions in two very different contexts. Pulmonary hypertension here means raised pressure in the lung arteries specifically, not the more familiar high blood pressure of the body's arteries.
What interactions and effects are known?
Nitrates for chest pain act through the same messenger and can cause a severe fall in blood pressure when combined with this drug, so the pairing is contraindicated. Riociguat, used for lung artery disease, is contraindicated for the same reason, and alpha blockers can add to the drop in pressure. Headache, flushing, blocked nose and indigestion are common and follow directly from vessels relaxing elsewhere in the body. A distinctive and harmless side effect is a blue tinge to vision or increased brightness, caused by weak action on a related enzyme in the retina. Rare reports include sudden hearing loss, a form of optic nerve damage from reduced blood flow, and a prolonged erection, which can damage the tissue permanently if blood flow is not restored.
Also known as
Used in a sentence
Sildenafil was prescribed for the patient after an evaluation for erectile dysfunction.
Good to know
Sildenafil and tadalafil belong to the same class but differ in how quickly they take effect and how long the effect lasts, which influences which one is selected in a given situation.