Glyceryl trinitrate (GTN): how to use the spray or tablets for angina

GTN is a nitrate. The NHS says it widens blood vessels, which increases blood supply to the heart and reduces chest pain from angina. It is used both to stop an angina attack and to prevent pain before an activity you know will bring it on. Patches are also used in hospitals to make veins more visible for infusions, and a rectal ointment treats anal fissures.

The NHS notes that the tablets, spray and patches can be bought from pharmacies without a prescription, whereas the rectal ointment needs one. Most adults aged 18 and over can use GTN; it is not used in children.

Always sit down first. For the spray, hold the nozzle in front of your mouth with a finger on the button, take a deep breath, open your mouth, lift your tongue and press once firmly to spray under the tongue. For tablets, place one under the tongue, close your mouth and let it dissolve slowly; do not suck, chew or swallow it.

For an attack, one dose is one tablet or one to two sprays, and the maximum is two doses, spaced five minutes apart. Before exertion, take one tablet or one to two sprays shortly before you start; either form typically works within one to three minutes.

• Sit down before every dose.

• Spray or place the tablet under the tongue only.

• Never take more than two doses for one attack.

• Keep spare unopened bottles for when the open one expires.

The NHS says call 999 if you have taken 2 doses of GTN and you are still in pain 5 minutes after the second dose, your pain is getting worse, or you feel unwell. These can indicate a heart attack. Do not drive yourself to hospital.

Also call 999 if your lips, tongue, gums, face or skin turn blue or grey after taking too much, or if you have chest pain that differs from your usual angina.

The NHS lists these reasons GTN may not be suitable: an allergic reaction to GTN or other medicines, a recent brain or head injury or bleeding in the brain, raised pressure in the head, a previous stroke, heart conditions beyond angina, a recent heart attack or heart failure, low blood pressure, anaemia and glaucoma.

Tell your doctor or pharmacist if any of these apply before using it.

Headaches are very common but usually go away after a few days if you use GTN regularly. Dizziness, weakness, tiredness and drowsiness mean you should stop and rest; nausea usually passes; and facial flushing typically subsides within hours to days. Do not drive if you feel dizzy, weak or tired.

Medicines for erectile dysfunction, including alprostadil, avanafil, sildenafil, tadalafil and vardenafil, can lower blood pressure too far and must not be combined with GTN. Mention heparin and similar medicines, antidepressants, blood pressure and heart medicines, ergot-containing migraine medicines and apomorphine. Paracetamol is fine, but avoid ibuprofen, aspirin and naproxen, which may worsen angina. Medicines that dry the mouth can reduce how well tablets dissolve, so a spray may suit better.

Alcohol increases the blood-pressure-lowering effect, so it can make you feel dizzy, faint or sleepy. The NHS says GTN is not harmful in pregnancy, although it is usually unnecessary then, and angina in pregnancy should be assessed by an obstetrician and a cardiologist. You can use it while breastfeeding if your baby is healthy, and there is no evidence it affects fertility.

If you have been taking GTN regularly and stop, your angina may worsen, which raises the chance of a heart attack or stroke, so speak to your doctor before stopping. Long-term angina prevention may use long-acting nitrates, beta blockers, calcium channel blockers, nicorandil or ranolazine.