Sildenafil: what it does and what to watch
Also known as: PDE5 inhibitor · Phosphodiesterase type 5 (PDE5) inhibitor · Updated 2026-08-19 · Educational — not medical advice
In plain language: During sexual arousal, the body releases a signaling molecule that widens blood vessels in specific tissue, but a natural enzyme called PDE5 normally breaks that signal down fairly quickly. This medicine blocks that enzyme, allowing the vessel-widening signal to last longer and work more effectively, which improves blood flow into the tissue involved. It doesn't create arousal on its own and depends on that underlying signal already being triggered, which is also why it widens blood vessels elsewhere in the body and can interact seriously with certain other blood-vessel-widening medicines.
What it's used for
- Erectile dysfunction
- Pulmonary arterial hypertension, at different doses and under specialist supervision
Common side effects
- Headache
- Flushing
- Nasal congestion
- Indigestion
- Dizziness
Get medical help if…
A dangerous drop in blood pressure if combined with nitrate medicines, which can be life-threatening and is an absolute reason to avoid this combination
Sudden vision changes, including loss of vision in one eye, which needs urgent medical evaluation
Sudden hearing loss or ringing in the ears, which also needs urgent evaluation
A prolonged, painful erection lasting several hours, which requires urgent medical attention
Chest pain or symptoms of significant cardiac strain during sexual activity, particularly relevant for people with underlying heart disease
Lab values it can move
Few guides tell you this part: medicines show up in blood work. If you take Sildenafil and a value below looks off on your report, this may be part of the story — never stop a medicine without your doctor.
Meaningful liver effects are uncommon at typical doses; rare, mild, transient liver enzyme changes have occasionally been reported and are generally not a routine monitoring concern for most people.
SGPT (ALT)Good to know
- Never take this medicine with nitrate medicines, including some used for chest pain, in any form, since the combination can cause a sudden and dangerous drop in blood pressure
- People with significant heart disease should discuss whether their heart is fit enough for the physical exertion of sexual activity before using this medicine, since the medicine itself doesn't create that assessment
- Sudden vision or hearing changes after taking this medicine are treated as urgent, not something to wait out
- Grapefruit juice and certain other medicines can raise blood levels of this medicine and are worth checking with a pharmacist
- This medicine does not protect against sexually transmitted infections and isn't a substitute for other appropriate precautions
- Alpha-blockers — medicines for prostate symptoms or blood pressure such as tamsulosin — can combine with this medicine to drop blood pressure enough to cause fainting; the prescriber needs to know about any alpha-blocker before the two are used together.
Questions people ask
Why is combining this with nitrate medicines so dangerous?
Both this medicine and nitrate medicines widen blood vessels through related pathways, and taking them together can amplify that effect far beyond what either produces alone, causing blood pressure to fall suddenly and severely. This combination has been linked to serious, sometimes fatal drops in blood pressure, so it's treated as an absolute rule, not a caution, with no safe way to combine them, even with a gap between doses.
Why would sudden vision or hearing changes need urgent care?
Rare cases of sudden vision loss in one eye, linked to reduced blood flow to part of the optic nerve, and sudden hearing loss have both been reported in association with this class of medicine, though a direct cause isn't fully settled. Because these changes can be signs of a serious, time-sensitive problem regardless of the cause, they're treated as an emergency, and the medicine is stopped while it's evaluated.
Does someone need a heart check before using this medicine?
The medicine itself is generally well tolerated, but sexual activity places a physical demand on the heart similar to moderate exercise, and for people with significant, unstable, or untreated heart disease, that demand can be risky independent of the medicine. This is why a broader conversation about heart health and exercise tolerance is a sensible part of starting this medicine for anyone with a relevant cardiac history.
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