Isosorbide Mononitrate — NZ Medication Guide

What is Isosorbide Mononitrate?

Isosorbide Mononitrate (generic name: isosorbide mononitrate) is a medication used primarily for prevention and long-term management of angina pectoris in coronary artery disease. Isosorbide mononitrate modified-release tablets (Imdur) are funded by Pharmac on the Pharmaceutical Schedule without Special Authority.

In New Zealand, Imdur (isosorbide mononitrate 60mg modified-release tablets) is the primary funded form. The Heart Foundation of New Zealand recommends long-acting nitrates as part of symptomatic management of angina when beta-blockers or calcium channel blockers are insufficient or not tolerated. Tolerance prevention (nitrate-free interval) is essential for maintaining efficacy.

What is Isosorbide Mononitrate Used For?

Isosorbide Mononitrate is prescribed by healthcare professionals in New Zealand for the following indications:

• Long-term prevention of angina attacks
• Chronic stable angina in coronary artery disease
• Adjunct therapy in heart failure
• Secondary prevention of ischaemic heart disease

How Does Isosorbide Mononitrate Work?

Isosorbide mononitrate is an organic nitrate that releases nitric oxide in vascular smooth muscle. This causes venodilation (reducing cardiac preload) and coronary artery dilation, decreasing the heart’s workload and oxygen demand while improving coronary blood flow. Unlike GTN, isosorbide mononitrate is used for prevention of angina rather than acute relief.

How to Take Isosorbide Mononitrate

Take isosorbide mononitrate exactly as prescribed, usually once or twice daily. Modified-release (MR) preparations are typically taken once in the morning. To prevent nitrate tolerance, a nitrate-free period of 8–12 hours per day is essential — your doctor will schedule your doses to allow for this. Do not crush or chew modified-release tablets. Take with or without food. Do not stop suddenly without consulting your doctor.

Common Side Effects

Like all medications, Isosorbide Mononitrate may cause side effects. Common side effects include:

• Headache (very common, especially at treatment initiation)
• Flushing and feeling of warmth
• Dizziness or lightheadedness
• Low blood pressure (hypotension)
• Nausea
• Palpitations (fast heartbeat)

Serious Side Effects — Seek Medical Attention

Contact your doctor or call 111 immediately if you experience any of the following serious effects:

• Severe hypotension — fainting, collapse
• Methaemoglobinaemia (rare) — blue discolouration of lips/skin
• Paradoxical angina worsening with excessive doses
• Syncope on standing (orthostatic hypotension)

Drug Interactions

Always inform your prescriber and pharmacist of all medications, supplements, and herbal products you are taking. Notable interactions with Isosorbide Mononitrate include:

• Phosphodiesterase-5 inhibitors (sildenafil, tadalafil) — CONTRAINDICATED; severe hypotension
• Other antihypertensives and vasodilators — additive hypotensive effect
• Alcohol — enhanced hypotension
• Calcium channel blockers — enhanced vasodilatory effect
• Dihydroergotamine — reduced nitrate efficacy

New Zealand Prescribing Information

In New Zealand, Imdur (isosorbide mononitrate 60mg modified-release tablets) is the primary funded form. The Heart Foundation of New Zealand recommends long-acting nitrates as part of symptomatic management of angina when beta-blockers or calcium channel blockers are insufficient or not tolerated. Tolerance prevention (nitrate-free interval) is essential for maintaining efficacy.

Frequently Asked Questions

Why do I need a nitrate-free period?

Continuous exposure to nitrate medications causes the blood vessels to become less responsive — a phenomenon called tolerance. To prevent this, your doses are timed to allow an 8–12 hour period each day without nitrate in your system. Typically this is overnight. Never adjust your dosing schedule without consulting your doctor or pharmacist.

Is isosorbide mononitrate the same as GTN spray?

No. GTN spray is for acute relief of an angina attack — it works within minutes. Isosorbide mononitrate is taken regularly as a preventive medication to reduce the frequency of angina attacks. You should still have GTN spray available for acute episodes even while taking isosorbide mononitrate.

References & Further Information

The following New Zealand and international resources were used to inform this page:

• New Zealand Formulary — Isosorbide Mononitrate
• Pharmac — Funded Medicines Schedule
• BPAC NZ — Stable Angina Management
• Medsafe — Imdur Data Sheet

Reviewed by a Registered Pharmacist — KiwiMeds New Zealand