Sildenafil side effects: what’s common, what’s rare and what needs urgent care

This page is medicine information rather than an advert. Sildenafil is a prescription only medicine in the UK at every strength except the pharmacy product described in our guide to Viagra Connect and pharmacy medicines. Whether it is suitable for you is a decision for a prescriber or pharmacist after an assessment.

Why the side effects look the way they do

Sildenafil blocks an enzyme called PDE5, which lets the natural signal that widens penile arteries persist. That enzyme is not only in the penis, and the medicine is not perfectly selective. Widened vessels elsewhere produce most of the common effects; a small crossover onto a related enzyme in the retina produces the visual ones.

Understanding that helps: most of what men notice is the mechanism working, not something going wrong.

Common effects

From the UK product information ^[1]:

Very common (more than 1 in 10): headache.

Common (up to 1 in 10): flushing or hot flushes, dizziness, nausea, indigestion, nasal congestion, altered colour vision, visual disturbance, blurred vision.

Uncommon (up to 1 in 100): fast heartbeat, palpitations, raised blood pressure, low blood pressure.

Practical things men find help: taking the lowest dose that works rather than the highest available; not stacking it on top of a large fatty meal, which delays absorption and prolongs the whole experience; staying hydrated; and giving it a few separate attempts before deciding, since the first occasion carries a lot of expectation.

The rare effects worth knowing by name

These are uncommon, but each has a specific action attached, which is why they are worth reading properly rather than skimming.

Priapism

An erection that will not go down. The product information lists it as rare ^[1]. An erection lasting more than four hours is a medical emergency — go to A&E. Blood trapped in the erectile tissue becomes deoxygenated and the tissue can be permanently damaged, so the treatment window matters. Men with sickle cell disease, myeloma or leukaemia, or with an anatomical abnormality of the penis, are at higher risk and should raise this before starting.

Sudden loss of vision (NAION)

Non-arteritic anterior ischaemic optic neuropathy is listed as a rare adverse reaction ^[1]. It causes sudden, usually painless loss of vision in one eye. Stop the medicine and get immediate medical attention — eye casualty or A&E. Men who have previously had NAION in either eye must not take sildenafil at all.

Sudden hearing loss

Sudden decrease or loss of hearing, sometimes with tinnitus or dizziness, is a recognised rare effect. Same instruction: stop and seek immediate medical advice.

Serious cardiovascular events

Heart attack, dangerous heart rhythms and sudden cardiac death appear in the rare category ^[1]. In most reported cases the men had existing cardiovascular risk factors. This is the reason the assessment asks about heart disease, and the reason sex itself is assessed as physical exertion — our guide to sex after a heart attack covers how that risk is judged.

Allergic reactions

Rash, itching, swelling of the face, lips or throat, or difficulty breathing. Call 999 for airway symptoms.

The combinations to avoid

Absolute ^{[1, 2]}:

• Nitrates in any form — glyceryl trinitrate spray, tablets or patches, isosorbide mononitrate or dinitrate.
• Nicorandil.
• Amyl nitrite (“poppers”).
• Riociguat.

These combinations can cause a dangerous and sometimes fatal fall in blood pressure. The washout advice runs both ways: if you take sildenafil and then need a nitrate, the standard guidance is a 24-hour gap for short-acting PDE5 inhibitors. Anyone who carries a GTN spray must tell the prescriber.

Requires care rather than avoidance: alpha blockers for blood pressure or prostate symptoms — stabilise on the alpha blocker first and start sildenafil at the lowest dose. Ritonavir, ketoconazole, itraconazole, erythromycin and clarithromycin all raise sildenafil levels and may mean a lower dose. Grapefruit juice has a small effect in the same direction.

When sildenafil is not suitable at all

The product information lists contraindications including severe heart disease where sexual activity is inadvisable, recent heart attack or stroke, blood pressure below 90/50, severe liver impairment, previous NAION, and hereditary degenerative retinal disorders such as retinitis pigmentosa ^[1].

Reporting side effects

Any suspected side effect can be reported to the MHRA through the Yellow Card scheme ^[4], by patients as well as clinicians. Reports are how new safety signals get found.

If the side effects are the problem

Do not simply stop and assume ED treatment is not for you. A lower dose, a different PDE5 inhibitor with a different profile, or a non-tablet option may suit better — tadalafil has a longer window and a different side effect pattern, as our sildenafil vs tadalafil comparison sets out. Speak to whoever prescribed it.

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.