Sildenafil and Nitrates Interaction

How This Interaction Works (Mechanism)

Nitrates release nitric oxide (NO), which activates guanylate cyclase to produce cGMP, causing smooth muscle relaxation and vasodilation. Sildenafil inhibits phosphodiesterase-5 (PDE5), the enzyme that breaks down cGMP. When both drugs are present, cGMP accumulates massively in vascular smooth muscle, causing profound systemic vasodilation and a catastrophic drop in blood pressure that cannot be easily reversed.

Source: VIAGRA label, Contraindications section 4.1 and Drug Interactions section 7.1, linked in full at the end of this page.

Why this one is a contraindication rather than a caution

Most entries in a drug interaction database are cautions: use with monitoring, adjust the dose, watch for a symptom. This one is not. The VIAGRA label lists nitrates under Contraindications, which is the strongest category a label has, and the wording covers "nitric oxide donors, such as organic nitrates or organic nitrites in any form".

The phrase "in any form" is doing real work. It covers nitroglycerin as a tablet under the tongue, as a spray, as a patch and as an ointment; isosorbide mononitrate and dinitrate taken regularly as tablets; nitroglycerin given intravenously in hospital; and amyl or alkyl nitrites sold recreationally as poppers, which are chemically the same class and are the route by which a large share of real cases occur. A patch removed that morning is not a past exposure in the way people assume.

The label also contraindicates use with guanylate cyclase stimulators such as riociguat, a pulmonary hypertension drug, for the same underlying reason.

The mechanism, and why the drop is hard to reverse

Nitrates work by releasing nitric oxide, which activates the enzyme guanylate cyclase and raises levels of cyclic GMP inside vascular smooth muscle. Cyclic GMP relaxes that muscle, the vessels widen, and blood pressure falls. That is the intended effect and it is how a nitrate relieves angina.

Sildenafil inhibits phosphodiesterase type 5, the enzyme whose job is to break cyclic GMP down. Taken alone that produces a modest and generally well-tolerated fall in blood pressure. Taken with a nitrate, one drug is pushing cyclic GMP production up while the other prevents its removal, and the label describes the result as sildenafil potentiating the hypotensive effect of nitrates.

The reason this is more serious than the phrase "low blood pressure" suggests is that the usual emergency correction is unavailable. A profound drop in pressure is ordinarily treated with fluids and vasopressors, and those act against a system that is being held open by an accumulation the body cannot clear quickly. The pressure has to come back largely as the drugs wear off.

How quickly it happens, and how long the risk lasts

Sildenafil is absorbed rapidly, with peak plasma concentrations typically reached within about an hour of a dose taken on an empty stomach, and a fatty meal delays that. The interaction is therefore an immediate one: a nitrate taken during that window can produce a fall in pressure within minutes.

The duration is where the honest answer differs from the widely repeated one. The label states directly that after a patient has taken sildenafil it is unknown when a nitrate can safely be given. The 24-hour figure that appears in most consumer articles, and previously on this page, comes from clinical consensus rather than from the label, and the longer figure quoted for tadalafil reflects its longer half-life rather than a separately established safe point.

The practical reading is that these numbers describe when the question becomes reasonable to ask, not a line after which the combination is proven safe. In an emergency it is a decision for the treating clinician with the timing in front of them, which is why the next section matters more than the number.

Who is at higher risk than the average reader

Anyone prescribed a nitrate regularly is by definition in the highest-risk group, because a regular nitrate prescription indicates coronary disease. The VIAGRA label adds a caution that sits alongside the interaction rather than inside it: patients should not use sildenafil if sexual activity is inadvisable because of their cardiovascular status. That is a separate judgement from the drug interaction and needs a doctor.

Two groups are at risk without knowing they are. The first is people who carry nitroglycerin for occasional angina rather than taking it daily, who may not think of themselves as being on a nitrate at all. The second is recreational users of poppers, who are frequently unaware that the substance is chemically a nitrite and falls squarely inside the contraindication.

Sildenafil exposure is also raised substantially by strong CYP3A4 inhibitors, which the label addresses with specific dose restrictions, including a maximum single dose of 25 mg in any 48-hour period for patients taking ritonavir. Higher exposure means a larger blood pressure effect.

If you have already taken both

Treat this as an emergency, and be specific about what you say.

Symptoms of a severe drop in blood pressure include dizziness or light-headedness, fainting or near-fainting, sudden severe weakness, blurred vision, nausea, and cold clammy skin. Call emergency services. If the person can be laid down with their legs raised while waiting, that helps a little; nothing else available at home does.

The most important action is to tell responders explicitly that a PDE5 inhibitor has been taken, and roughly when. The reason is direct: chest pain is ordinarily treated with nitroglycerin, and a person who is having chest pain after taking sildenafil can be given the exact drug that will make the situation worse if nobody knows. This is the reason the advice to carry that information exists, and it applies to sildenafil, tadalafil, vardenafil and avanafil alike.

If a nitrate has been taken and sildenafil has not yet been swallowed, do not take it, and do not treat a small dose as a small risk. If both have been taken and there are no symptoms, contact a doctor or emergency line for advice rather than assuming the window has passed.

Where every claim on this page comes from

Both labels below are FDA-approved manufacturer labels, linked by the identifier of the exact version used here. Where this page reports the commonly quoted 24 and 48 hour intervals, it attributes them to cardiology consensus guidance rather than to these labels, because the labels do not establish them.

• VIAGRA (sildenafil citrate) prescribing information, Pfizer Labs, label version of 15 December 2017. Retrieved from the FDA label database via openFDA and linked here by its DailyMed set identifier, so the exact version quoted can be opened and checked.
• NITROSTAT (nitroglycerin) prescribing information, Parke-Davis, Div of Pfizer, label version of 6 November 2025. Retrieved from the FDA label database via openFDA and linked here by its DailyMed set identifier, so the exact version quoted can be opened and checked.

Labels retrieved 29 August 2026. A product label is revised over time: the version linked above is the one this page was written against, and a newer revision may differ. Where this page states something that is not in a label, it says so and names what it is relying on instead.

Sources and standards behind this page

Drug coverage in the United States is governed by published rules rather than by any one insurer's summary, and the sources below are the primary ones. They are linked so a reader can check a claim without taking this page's word for it.

• FDA Orange Book publishes the therapeutic-equivalence ratings that decide whether a generic may be substituted for a brand-name drug.
• FDA generic drug facts sets out what "generic equivalent" means and what a manufacturer must prove.
• CMS prescription drug coverage defines the rules a Medicare drug plan formulary must follow and how it may change mid-year.
• Medicare.gov drug coverage (Part D) states what a Part D plan is required to offer a member.
• CMS prescription drug appeals documents the coverage-determination and exception process when a drug is denied.
• Medicare Plan Compare is the official tool for checking what a specific plan charges for a specific drug at a specific pharmacy.
• Drugs@FDA is the federal database of approved drug products, which establishes whether a covered product exists and in what form.
• DailyMed carries the current FDA labeling for every marketed drug product, including approved strengths and forms.
• MedlinePlus drug information is the National Library of Medicine's consumer reference for what a medication is and how it is used.
• FTC report on pharmacy benefit managers documents how the middlemen between an insurer and a pharmacy shape what a plan actually covers.
• National Library of Medicine maintains the drug and health databases the sources above draw on.

This page describes how coverage and formularies work. It is not medical advice, and a reader's own plan documents and prescriber decide what applies to them.