Sildenafil side effects, and how likely each one is

Most men who take sildenafil don't get side effects. The ones who do usually find them mild, short-lived, and manageable by adjusting the dose.

Here's how often each one actually occurs, which ones mean stopping, and the handful that need urgent attention. If you're new to sildenafil for erectile dysfunction, start with how to take sildenafil and how long it lasts.

Get urgent medical help if

Your erection lasts more than four hours: This is called priapism. It's a surgical emergency, and waiting risks permanent damage. Go to A&E.

You suddenly lose vision, in one eye or both.

You suddenly lose hearing, or develop ringing in your ears.

You have chest pain: And if you're prescribed nitrates for angina, do not take them to relieve it. Tell the paramedic or doctor that you've taken sildenafil.

These are rare. They're first because recognising them quickly is what matters.

How common is each side effect?

The UK product licence draws on 9,570 patients across 74 placebo-controlled studies.¹

Reactions are graded by how often they occur:

NAION has been reported since the medicine went on sale, so its exact frequency isn't known.

In one of the first large trials, headache, flushing, and indigestion were the most common side effects, affecting between 6% and 18% of men.²

How long do side effects last?

Usually a few hours, and they tend to fade as the drug clears your system.

Sildenafil's effects wear off over several hours, and side effects generally follow the same curve. A headache that starts an hour after taking it will typically have gone by the next morning.

If something persists for more than a day, or keeps happening every time you take it, that's worth raising with your doctor rather than putting up with.

Headaches, and why a lower dose often helps

Headache is the most frequent side effect, and it's often dose-related.

The licence data shows that going above the recommended dose doesn't improve how well sildenafil works, but does increase side effects, including headache, flushing, dizziness, indigestion, blocked nose, and visual changes.¹

Which means a lower dose is often the answer rather than putting up with it. Numan prescribes sildenafil at 25mg, 50mg, and 100mg. If you're on 100mg or 50mg and the headaches are the problem, speak to the clinical team about dropping down rather than stopping altogether.

When sildenafil isn't suitable

Some of these rule it out completely.

Never take sildenafil if you take:

• Nitrates, usually prescribed for angina or chest pain. Together they can cause a dangerous drop in blood pressure.

• Amyl nitrite or other poppers, for the same reason.

• Riociguat, a medicine for high blood pressure in the lungs.

Sildenafil is also unsuitable if you have:

• A severe heart or liver problem

• Recently had a stroke or heart attack, or have low blood pressure

• A rare inherited eye condition such as retinitis pigmentosa

• Ever lost vision through non-arteritic anterior ischaemic optic neuropathy (NAION)

This isn't a complete list, which is why sildenafil is prescription-only and why the assessment exists.

Sildenafil or tadalafil: which causes fewer side effects?

Broadly similar. A pooled analysis of 16 trials found comparable efficacy and safety profiles between sildenafil and tadalafil.³

The practical difference is duration rather than tolerability. If side effects are the deciding factor, the dose matters more than the choice of drug.

The numan take

Headache is the one most people get, it's usually dose-related, and a lower dose often solves it.

The two things worth remembering are that nitrates and poppers rule sildenafil out completely, and that an erection lasting more than four hours needs A&E rather than patience.

Read the Patient Information Leaflet that comes with your tablets, because it's specific to the product you've been given.

References

^{1 Sildenafil 50 mg film-coated tablets. medicines.org.uk. [accessed 13 October 2026] Available from: https://www.medicines.org.uk/emc/product/7141/smpc}

^{2 Goldstein I, Lue TF, Padma-Nathan H, Rosen RC, Steers WD, Wicker PA. Oral sildenafil in the treatment of erectile dysfunction. N Engl J Med. 1998;338(20):1397–1404.} 

^{3 Gong B, Ma M, Xie W, Yang X, Huang Y, Sun T, et al. Direct comparison of tadalafil with sildenafil for the treatment of erectile dysfunction: a systematic review and meta-analysis. Int Urol Nephrol. 2017;49(10):1731–40.}