Sildenafil and Nitrates: Why This Combination Is Dangerous

Sildenafil and nitrates must never be taken together. This is an absolute contraindication, not a caution or a 'use with care' warning. The combination can cause a severe, potentially fatal drop in blood pressure. UK regulatory guidance from the MHRA and the British National Formulary (BNF) is unambiguous on this point.

Why the combination is so dangerous#

Both drugs act on the same chemical pathway, but from opposite ends. Nitrates release nitric oxide, which triggers an enzyme called guanylate cyclase to produce cyclic guanosine monophosphate (cGMP). cGMP relaxes the walls of blood vessels, causing them to widen and blood pressure to fall. This is how nitrates relieve angina.

Sildenafil works by blocking PDE5, the enzyme that normally breaks cGMP down. With PDE5 inhibited, cGMP accumulates and vessel dilation is prolonged. Take both drugs together and you get a double hit: nitrates flooding the system with cGMP while sildenafil stops it clearing. The combined effect can cause blood pressure to fall dramatically — enough to cause blackout, a heart attack from inadequate coronary perfusion, or stroke.

Which nitrates are affected#

All nitrates carry this contraindication, not just the GTN spray most people think of. The table below covers the main ones prescribed in the UK.

Nicorandil deserves a specific mention because many UK patients who take it for angina do not realise it acts like a nitrate. It opens potassium channels but also releases nitric oxide in the same way, producing the same dangerous interaction with sildenafil. If you take nicorandil, the same absolute prohibition applies.

Amyl nitrite ('poppers') is used recreationally in some communities and is widely available. The interaction with sildenafil is just as dangerous. Both cause profound vasodilation and the combination can produce circulatory collapse within minutes. If you have taken sildenafil, avoid poppers for at least 24 hours.

How long you need to wait: the 24-hour versus 72-hour question#

The BNF states that nitrates must not be used within at least 24 hours of taking sildenafil. That figure is the licensed minimum safety threshold. In a clinical study by Arruda-Olson and colleagues assessing the cardiovascular effects of sildenafil in men with coronary artery disease, researchers required a 72-hour nitrate-free window before giving sildenafil — reflecting the concern that cardiac patients often have fragile circulatory reserve. This research interval does not replace BNF-licensed guidance; if you have coronary disease or heart failure, speak to your cardiologist before making any decisions about nitrate-free windows.

Sildenafil itself has a half-life of around 4 hours and is largely cleared within 24 hours. Tadalafil (Cialis) has a much longer half-life of approximately 17.5 hours. Because of this, the nitrate-free window after tadalafil is longer; the BNF contraindicates tadalafil with nitrates, and you should consult the current BNF entry or your prescriber for the specific interval that applies to your situation. Vardenafil and avanafil sit closer to sildenafil in half-life, but all four licensed PDE5 inhibitors carry the same absolute contraindication with nitrates.

There is no licensed scenario in which a man on regular long-term nitrate therapy can temporarily stop his nitrates to use a PDE5 inhibitor. Stopping nitrate therapy abruptly to create a drug-free window is not recommended without direct supervision from a cardiologist. The angina risk during the nitrate-free period is real.

If you accidentally take both#

If you have taken sildenafil and then used a nitrate (or vice versa), act quickly.

• Call 999 immediately if you feel faint, collapse, experience chest pain, or lose consciousness. Tell the dispatcher you have taken both drugs.
• Call NHS 111 if you feel dizzy or lightheaded but are conscious and stable. They can advise on monitoring and when to escalate.
• Do not drive yourself to hospital.
• Do not take a second dose of either drug trying to 'cancel' the effect.

Hospital treatment for sildenafil-induced hypotension typically involves intravenous fluids and careful vasopressor support. The usual response to low blood pressure, which is giving a nitrate to reduce cardiac workload, is contraindicated in this situation. Tell any treating clinician exactly what you have taken and when.

What cardiac patients can use instead#

Men on long-term nitrates who want to treat erectile dysfunction have several options that do not interact with nitrates.

Alprostadil (Caverject injection or MUSE intraurethral pellet) works through a completely different pathway and has no interaction with nitrates. It is effective but requires more preparation than a tablet.

Vacuum erection devices are non-pharmacological and carry no interaction risk. They are available on NHS prescription for some men and work independently of any medication.

The right route is a conversation with your GP or cardiologist before trying anything. UK GPs use cardiac risk stratification criteria (based on Princeton Consensus-style guidance) to decide whether a patient's heart condition is stable enough to consider any ED treatment at all. If your angina is well-controlled and infrequent, there may be options. If it is active or unstable, treating the cardiac condition takes priority.

A useful starting point is telling your GP or cardiologist directly: 'I have erectile dysfunction and I want to discuss what options are safe given my nitrate treatment.' Many men avoid the conversation, but GPs are used to it and it is the only way to get an answer tailored to your cardiac status.

Sildenafil bought over the counter#

Since 2018, sildenafil 50 mg (Viagra Connect) has been available without a prescription from UK pharmacies. This is convenient, but it creates a safety gap. A pharmacist should ask about nitrate use before selling it as part of their professional obligations, but this check may not always occur, and patients may not recognise nicorandil as a nitrate. If you are on any heart medication, it is worth checking with the pharmacist or your GP before buying sildenafil over the counter, even if the pack says no prescription needed.

You can report any adverse reactions from drug interactions, including accidental sildenafil-nitrate combinations, to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

If you do not take nitrates and are looking for a safe, clinically reviewed ED treatment, Totiva offers a free online consultation where a UK clinician will review your full medication list before prescribing. You can start an ED consultation at Totiva to find the right option for your situation.