Isosorbide Mononitrate
Also known as: ISMN, Imdur, Elantan
What it's for
Isosorbide mononitrate prevents angina (chest pain from heart disease). It relaxes blood vessels, making it easier for blood to flow. It does not stop an angina attack once started.
Comes as: Tablets (standard or modified-release)
How to take it
- Standard tablets: Usually 20mg 2 or 3 times daily.
- Modified-release: Usually 25mg to 120mg once daily in the morning.
- Take as prescribed to maintain protection throughout the day.
- Swallow modified-release tablets whole.
Doses here are typical. Always follow the instructions from your prescriber or on the packet — they come first.
Common side effects
- Headache (very common, usually improves after a few days)
- Feeling dizzy or lightheaded
- Flushing
- Feeling sick
The leaflet inside the packet lists all known side effects.
Important safety points
- Headaches are common at first but usually get better
- Do not use with medicines for erectile dysfunction (Viagra, etc.)
- May make you dizzy - take care when driving
- Do not stop suddenly if used regularly
Get medical help urgently if…
- You have chest pain that your GTN spray does not relieve
- You feel very dizzy or faint
- You have a severe headache with stiff neck
- You have signs of an allergic reaction
Call NHS 111 for urgent advice, or 999 if someone is seriously unwell.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.