When should you not use nitrates? A definitive guide to contraindications and risks

October 11, 2025 •

4 min read

Nitrates are a cornerstone for managing angina, with millions relying on them for chest pain relief. However, their powerful vasodilatory effects make understanding when should you not use nitrates a critical safety concern, as certain pre-existing conditions and drug combinations can lead to life-threatening complications.

Quick Summary

Nitrates must be avoided with PDE5 inhibitors, severe hypotension, hypertrophic cardiomyopathy, and increased intracranial pressure. Extreme caution is warranted in cases of severe anemia and right ventricular infarction due to high-risk side effects.

Key Points

PDE5 Inhibitors: Never combine nitrates with erectile dysfunction medications like sildenafil (Viagra) due to the risk of severe, life-threatening hypotension.
Hypertrophic Cardiomyopathy (HOCM): Avoid nitrates in HOCM as they can worsen outflow tract obstruction and lead to low blood pressure.
Low Blood Pressure: Do not use nitrates if you have severe hypotension (e.g., SBP < 90 mmHg) as they will lower your blood pressure further.
Drug and Alcohol Interactions: Avoid or limit alcohol consumption and inform your doctor of all other medications, as they can have additive hypotensive effects.
Increased Intracranial Pressure: In conditions like head injury or cerebral hemorrhage, nitrates are contraindicated as they can aggravate intracranial pressure.
Severe Anemia: Be cautious with nitrates if you have severe anemia, as they can cause methemoglobinemia and worsen oxygen delivery.
Right Ventricular Infarction: The use of nitrates in this condition requires extreme caution and careful monitoring due to preload dependence.
Consult a Professional: Always consult your doctor or pharmacist about your full medical history and all current medications before starting or continuing nitrate therapy.

Absolute Contraindications: Severe Health Risks

For some conditions, using nitrates is strictly forbidden, as the risk of a severe, life-threatening event is extremely high. These are known as absolute contraindications and require complete avoidance of nitrates in any form.

Use with Phosphodiesterase-5 (PDE5) Inhibitors

One of the most dangerous interactions involves combining nitrates with PDE5 inhibitors, a class of drugs used for erectile dysfunction (ED) and pulmonary hypertension. Both drug classes are powerful vasodilators, meaning they relax and widen blood vessels. When taken together, their effects on lowering blood pressure are dangerously amplified.

Sildenafil (Viagra), Vardenafil (Levitra): A combination with nitrates can lead to a drastic, life-threatening drop in blood pressure, potentially causing cardiogenic shock, myocardial ischemia, or even death. It is crucial to allow at least 24 hours to pass between taking these medications and using a nitrate.
Tadalafil (Cialis): Due to its longer half-life, a waiting period of at least 48 hours is recommended before administering a nitrate.

Use with Soluble Guanylate Cyclase (sGC) Stimulators

Another critical drug interaction is with sGC stimulators like riociguat (Adempas), a medication used to treat pulmonary hypertension. Similar to the PDE5 inhibitors, this combination poses a severe risk of causing a profound drop in blood pressure and should be strictly avoided.

Hypertrophic Obstructive Cardiomyopathy (HOCM)

HOCM is a condition where the heart muscle becomes abnormally thick, often causing the septum to obstruct blood flow out of the heart. Nitrates reduce preload (the volume of blood returning to the heart) which can worsen the outflow tract obstruction. This can lead to a further decrease in cardiac output, severe hypotension, and syncope.

Severe Hypotension

Patients with significantly low blood pressure (e.g., systolic blood pressure < 90 mmHg) should not use nitrates. The powerful vasodilating effects can cause an even more severe, potentially fatal, drop in blood pressure. This risk is especially pronounced in patients who are volume-depleted or have other hypotensive risk factors.

Conditions Requiring Extreme Caution

These conditions are not always absolute contraindications, but they require careful clinical consideration and close monitoring due to the potential for significant adverse effects.

Right Ventricular (RV) Infarction

In an RV infarction, the right side of the heart is damaged. The right ventricle is particularly dependent on preload to maintain cardiac output. Nitrates, by decreasing preload, can cause a severe reduction in blood pressure and cardiac output in these patients. While not an absolute contraindication in all cases, it demands careful clinical or hemodynamic monitoring and is often avoided.

Increased Intracranial Pressure (ICP)

Nitrates can increase intracranial pressure by dilating the cranial capacitance vessels. This effect can be particularly dangerous for patients with conditions like cerebral hemorrhage or traumatic brain injury. Using nitrates in these patients can aggravate the pressure, potentially worsening neurological outcomes.

Severe Anemia

In cases of severe anemia, nitrates should be used with caution, if at all. Large doses of nitrates can cause methemoglobinemia, a condition where hemoglobin is converted to methemoglobin, which cannot carry oxygen effectively. This exacerbates the existing anemia and can impair oxygen delivery to tissues.

Avoiding Harmful Drug Interactions

Beyond the specific drug classes mentioned as absolute contraindications, other substances can interact negatively with nitrates, increasing the risk of adverse effects. It is vital to inform your healthcare provider of all medications, supplements, and lifestyle habits.

Alcohol: Consuming alcohol while on nitrate therapy can significantly increase the risk of hypotension, dizziness, and fainting due to its additive vasodilatory effects.
Other Blood Pressure Medications: While not always contraindicated, other antihypertensive drugs like calcium channel blockers and beta-blockers can have additive blood pressure-lowering effects with nitrates. Careful dosage adjustment and monitoring are often required.
Tricyclic Antidepressants: Some older antidepressants can cause dry mouth, which may hinder the dissolution of sublingual nitroglycerin tablets, making them less effective.

Comparison of Contraindications for Nitrates


Condition / Drug	Nitroglycerin	Isosorbide Dinitrate	Isosorbide Mononitrate
PDE5 Inhibitors (e.g., Sildenafil)	Absolute Contraindication	Absolute Contraindication	Absolute Contraindication
Riociguat (sGC Stimulator)	Absolute Contraindication	Absolute Contraindication	Not specified, but caution advised based on mechanism
Severe Hypotension	Absolute Contraindication	Absolute Contraindication	Absolute Contraindication
Hypertrophic Cardiomyopathy	Avoid/Extreme Caution	Avoid/Extreme Caution	Avoid/Extreme Caution
Severe Anemia	Absolute Contraindication	Absolute Contraindication	Absolute Contraindication
Increased Intracranial Pressure	Absolute Contraindication	Not specified, but caution advised based on mechanism	Avoid/Extreme Caution
Right Ventricular Infarction	Avoid/Extreme Caution	Not specified, but caution advised based on preload dependence	Not specified, but caution advised based on preload dependence

Conclusion: Always Consult a Healthcare Provider

Nitrates are effective and necessary medications for many cardiovascular conditions, but their use is not without risk. Understanding when should you not use nitrates is paramount to patient safety and preventing serious adverse events. Combining nitrates with PDE5 inhibitors or sGC stimulators is unequivocally forbidden due to the risk of fatal hypotension. Furthermore, conditions such as hypertrophic cardiomyopathy, severe anemia, and increased intracranial pressure represent critical contraindications or situations demanding extreme caution. Drug interactions with other hypotensive agents and alcohol further underscore the need for vigilance. Patients must always provide a complete medical history, including all medications and supplements, to their healthcare provider to ensure safe and effective treatment. Never adjust or cease nitrate therapy without medical supervision.

For more information on cardiovascular medications and safety, please refer to the FDA Drug Labeling for Nitrates.

Frequently Asked Questions

Taking Viagra (sildenafil) and a nitrate together is an absolute contraindication and can cause a severe, life-threatening drop in blood pressure. Both medications are potent vasodilators, and their combined effect can lead to cardiac shock or a heart attack.

No, you should not use nitrates if you have severe hypotension (low blood pressure), typically defined as a systolic blood pressure below 90 mmHg. The vasodilatory action of nitrates will further decrease blood pressure and can cause serious complications like fainting or shock.

Nitrates should be avoided in patients with hypertrophic obstructive cardiomyopathy. By reducing preload, nitrates can worsen the obstruction of blood flow from the heart, leading to increased symptoms and dangerous reductions in cardiac output.

Nitrates can increase intracranial pressure (ICP) by dilating blood vessels in the brain. This is particularly dangerous after a head injury or with conditions like cerebral hemorrhage, as it can worsen the pressure and potentially harm neurological function.

No, it is not recommended. Alcohol has vasodilating effects that can be additive to those of nitrates, increasing the risk of a significant drop in blood pressure, dizziness, and fainting.

Nitrates are often contraindicated in patients with severe anemia. The risk is the development of methemoglobinemia, a condition where oxygen-carrying capacity is reduced, which can be especially harmful to someone who is already anemic.

The safe time interval depends on the specific PDE5 inhibitor. For sildenafil and vardenafil, wait at least 24 hours. For tadalafil, a waiting period of at least 48 hours is recommended due to its longer duration of action.