How erectile dysfunction tablets work, and how they differ

This page explains how this class of medicine works and how the four licensed options differ. It is information about prescription medicines, not advice about whether any of them is right for you.

The mechanism

Arousal causes nerve endings in the penis to release nitric oxide. That triggers production of a signalling molecule, cyclic GMP, which relaxes the smooth muscle in the arteries so blood can fill the erectile tissue.

An enzyme called phosphodiesterase type 5 breaks cyclic GMP down, ending the erection.

PDE5 inhibitors block that enzyme. Cyclic GMP persists, the vessels stay relaxed for longer, and the erection is easier to achieve and maintain.

The consequence people most need to know: they do not cause erections. Without arousal there is no nitric oxide, no cyclic GMP, and nothing for the medicine to preserve. Every UK product information document states that sexual stimulation is required.

The four licensed in the UK

Sources: the respective UK Summaries of Product Characteristics ^[1][2][3].

Two points that are genuinely distinctive rather than marketing. Tadalafil demonstrated improvement in erectile function up to 36 hours after dosing, and is the only one of the four licensed for a once-daily regimen and for benign prostatic hyperplasia. Avanafil has the fastest licensed onset.

One correction worth making: “sildenafil lasts 4 to 6 hours” is a clinical convention, not product information. The SmPC gives a half-life of 3–5 hours and a time to peak concentration of 30–120 minutes, and does not state a duration of effect.

The contraindications, stated correctly

This is where most published material goes wrong.

Absolute, across the class

Nitrates, in any form. This includes GTN spray and patches, isosorbide mono- and dinitrate, amyl nitrite (“poppers”), and — frequently forgotten — nicorandil and molsidomine. The combination can cause a profound and dangerous drop in blood pressure.

Riociguat and other guanylate cyclase stimulators.

Previous loss of vision in one eye due to non-arteritic anterior ischaemic optic neuropathy (NAION).

Cardiovascular limits — and these differ

A common error is applying one medicine’s figures to another:

• Sildenafil (Viagra, POM): contraindicated with a “recent history” of stroke or myocardial infarction — the SmPC gives no number ^[1]
• Tadalafil: contraindicated with MI within the last 90 days, stroke within the last 6 months, NYHA class II or greater heart failure in the last 6 months, uncontrolled arrhythmias, hypotension below 90/50, or uncontrolled hypertension ^[2]

Any article attributing “90 days” to sildenafil is wrong.

Alpha blockers are NOT a contraindication

They are a precaution. Sildenafil’s product information advises stabilising on the alpha blocker first and considering a 25 mg starting dose. Tadalafil’s advises caution, and specifically that the combination of tadalafil and doxazosin is not recommended ^[2].

Describing alpha blockers as contraindicated is a frequent and unhelpful error, because it puts men off raising the subject at all.

Other cautions

Severe hepatic impairment; severe renal impairment; hypotension; hereditary degenerative retinal disorders such as retinitis pigmentosa; conditions predisposing to priapism such as sickle cell disease, myeloma and leukaemia; anatomical deformity of the penis. Avanafil additionally contraindicates potent CYP3A4 inhibitors such as ketoconazole, ritonavir and clarithromycin outright, which is stricter than the others ^[3].

Side effects

Common: headache, flushing, indigestion, nasal congestion, back and muscle aches, visual disturbance such as a blue tinge or light sensitivity.

Require urgent attention:

• An erection lasting more than four hours — priapism, a medical emergency
• Sudden loss of vision in one or both eyes
• Sudden hearing loss, with or without ringing and dizziness

Current tadalafil product information notes that observational data suggest an increased risk of acute NAION in men with erectile dysfunction exposed to PDE5 inhibitors, and also names central serous chorioretinopathy, most cases of which resolved after stopping ^[2].

Suspected side effects can be reported through the MHRA Yellow Card scheme ^[5].

Practical notes

They do not always work first time. Product information advises that several attempts on different occasions may be needed. Around three-quarters of men respond to an adequate dose used correctly.

Dose matters. Many apparent failures are an inadequate dose, or taking it with a heavy meal, or without sufficient arousal.

They treat the symptom. Whatever caused the problem is still there, which is the argument for assessment rather than purchase.

Safety information

These are prescription-only medicines in the UK, other than specific pharmacy products supplied under a pharmacist’s assessment. They require a proper consultation covering your cardiovascular history, current medicines and other conditions.

Do not obtain them from sellers that do not require a consultation. Between 2021 and 2025 UK enforcement seized around 20 million doses of illegal erectile dysfunction medicines, with 4.4 million in 2025 alone.

When to speak to a clinician

Seek medical advice before use if you:

• take nitrates in any form, including GTN spray, nicorandil or poppers
• take riociguat
• have had a heart attack or stroke recently, unstable angina, or heart failure
• have low blood pressure, or uncontrolled high blood pressure
• have significant liver or kidney impairment
• have a retinal condition, or have previously lost vision in one eye from NAION
• have sickle cell disease, myeloma or leukaemia
• become very breathless or get chest pain walking briskly for 20 minutes or climbing two flights of stairs

Attend A&E for an erection lasting more than four hours, or for sudden loss of vision or hearing.

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.