Sildenafil

What sildenafil is and how it works

Sildenafil is a prescription-only PDE5 inhibitor taken as needed to help achieve and maintain an erection sufficient for sexual activity. Sexual stimulation is still required for it to work. (1)

Take sildenafil only as your prescriber directs. The single most important safety rule is this: never take sildenafil if you use any nitrate medicine, in any form, even occasionally -- the combination can cause a severe, potentially life-threatening drop in blood pressure.

Mechanism of action: how sildenafil works

Sildenafil selectively inhibits phosphodiesterase type 5 (PDE5), the enzyme responsible for degradation of cyclic GMP in the corpus cavernosum, enhancing the nitric-oxide-mediated increase in blood flow that produces an erection in response to sexual stimulation. (1)

What sildenafil is used for

Before taking sildenafil

Do not take / not appropriate for

• Do not take sildenafil if you use any nitrate medicine in any form -- regularly or only occasionally -- including nitroglycerin, isosorbide dinitrate, or isosorbide mononitrate (often used for chest pain/angina). Combining the two can cause a severe, potentially fatal drop in blood pressure. (all countries) (1)
• Do not take sildenafil if you take a guanylate cyclase stimulator such as riociguat, for the same severe blood-pressure risk. (all countries) (1)
• Do not take sildenafil if you have had a serious allergic reaction to it or any of its ingredients. (all countries) (1)

Tell your clinician about

• Any heart condition, including whether sexual activity is safe for you -- your prescriber needs to confirm your heart can handle the physical exertion of sex.
• Whether you take an alpha-blocker medicine (often used for blood pressure or an enlarged prostate).
• Any history of an erection lasting longer than 4 hours, sickle cell disease, multiple myeloma, leukaemia, or a physical deformity of the penis (such as Peyronie's disease).
• Any history of sudden vision or hearing loss.
• Kidney or liver problems.
• All other medicines you take, including any nitrates you use only occasionally.

How to take sildenafil

• Take sildenafil as needed, roughly 1 hour before sexual activity -- it can work anywhere from about 30 minutes to 4 hours after taking it.
• Take no more than one dose in a 24-hour period.
• Sexual stimulation is still needed for sildenafil to work -- it does not cause an erection on its own.

Procedures and treatment changes

Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (3) (2) (4) (5) (6)

Missed dose

Sildenafil is taken as needed rather than on a fixed daily schedule, so a 'missed dose' does not apply in the usual sense -- take it only when needed, up to once per day, following your prescriber's instructions.

Overdose

Seek emergency medical help immediately if too much sildenafil has been taken, or if severe dizziness, fainting, or a prolonged erection occurs. (1)

Side effects

Side effects and their urgency vary; report severe, persistent, or concerning symptoms.

Serious warnings

Interactions

• Any nitrate medicine, in any form, even used occasionally, is a hard contraindication with sildenafil due to the risk of severe hypotension. (1)
• Guanylate cyclase stimulators such as riociguat are also contraindicated with sildenafil, for the same reason. (1)
• Alpha-blocker medicines (often used for blood pressure or an enlarged prostate) can cause an additive blood-pressure-lowering effect -- your prescriber will usually start you on a lower sildenafil dose if you take one. (1)
• Strong CYP3A4-inhibiting medicines (such as ritonavir, ketoconazole, itraconazole, or erythromycin) can raise sildenafil levels -- your prescriber may start you at a lower dose or limit how often you take it. (1)
• Combining sildenafil with other PDE5 inhibitors has not been studied and is not recommended. (1)

Pregnancy, breastfeeding, kidney/liver function, age

Monitoring

Before starting

• Cardiovascular assessment to confirm sexual activity is safe
• Review of all current medicines, especially nitrates and alpha-blockers
• Baseline vision and hearing history

During treatment

• Watching for any signs of chest pain or cardiovascular symptoms during sexual activity.
• Watching for sudden vision or hearing changes.
• Watching for an erection lasting more than 4 hours.

Storage

Store at 25C (77F); brief excursions between 15-30C (59-86F) are permitted. Keep out of reach of children.

Frequently asked questions

References

1. VIAGRA (sildenafil citrate) tablets -- full prescribing informationU.S. Food and Drug Administration / DailyMed (NIH), full prescribing information, Pfizer · United States · accessed 2026-07-29 · source 1
2. Viagra 25 mg/50 mg/100 mg film-coated tablets -- Summary of Product Characteristicselectronic medicines compendium (emc), United Kingdom -- Pfizer · United Kingdom · accessed 2026-07-29 · source 2
3. Sildenafil -- EU nationally authorised medicinal products for erectile dysfunctionEuropean Medicines Agency (EMA) -- sildenafil-class regulatory status · European Union · accessed 2026-07-29 · source 3
4. VIAGRA (sildenafil citrate) -- Product MonographHealth Canada product monograph · Canada · accessed 2026-07-29 · source 4
5. VIAGRA sildenafil (as citrate) 100 mg tablet blister pack -- ARTG registrationTherapeutic Goods Administration (TGA), Australia · Australia · accessed 2026-07-29 · source 5
6. VIAGRA (sildenafil citrate) -- New Zealand Data SheetMedsafe (New Zealand regulator) · New Zealand · accessed 2026-07-29 · source 6

Brand names by country

Single-ingredient sildenafil names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.

Priority countries