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Side effects of montelukast - Brand name: Singulair

Like all medicines, montelukast can cause side effects, although not everyone gets them.

Common side effects

These common side effects of montelukast happen in more than 1 in 100 people. There are things you can do to help cope with them:


Drink lots of fluids, such as water or squash, to avoid dehydration. Signs of dehydration include peeing less than usual or having dark strong-smelling pee. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.

If you take contraceptive pills and you have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice.

High temperature

Paracetamol will help to bring the temperature down. Drink plenty of fluids. See your doctor if it lasts for more than a few days.


Make sure you rest and drink plenty of fluids. Do not drink too much alcohol. Ask your pharmacist to recommend a painkiller.

Headaches should usually go away after the first week of taking montelukast. Talk to your doctor if headaches last longer than a week or are severe.

Stomach pain

Try to rest and relax. It can help to eat and drink slowly and have smaller and more frequent meals. Putting a heat pad or covered hot water bottle on your stomach may also help.

If you're in a lot of pain, speak to your pharmacist or doctor.

Feeling or being sick

Stick to simple meals and do not eat rich or spicy food. If you're being sick, try small frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark strong-smelling pee.

If you take contraceptive pills and you're being sick your contraception may not protect you from pregnancy. Check the pill packet for advice.

A mild rash

It may help to take an antihistamine, which you can buy from a pharmacy. Check with the pharmacist to see what type is right for you.

If the rash gets worse, speak to your doctor.

Speak to a doctor or pharmacist if the advice on how to cope does not help and a side effect is still bothering you or does not go away.

Serious side effects

Call your doctor or call 111 immediately if:

  • you notice mood changes and you become anxious, depressed or aggressive
  • you see things which are not there (hallucinations)
  • you're finding it harder than usual to concentrate or remember things
  • you have new problems with your sleep, or sleep problems that get worse, such as nightmares, sleepwalking or difficulty sleeping
  • your speech changes or you start stuttering
  • you have shaking or trembling in any part of your body
  • you have an unusual or fast heartbeat
  • you develop a combination of symptoms such as a flu-like illness, pins and needles or numbness of arms or legs, worsening of asthma symptoms, or a rash
  • the whites of your eyes turn yellow, or your skin turns yellow although this may be less obvious on brown or black skin – this can be a sign of liver problems

It's important to tell your family or friends that you're taking montelukast, and that it may sometimes affect your mood, sleep or behaviour. This is so they can help you notice any changes.

Immediate action required: Call 999 or go to A&E if:

  • you have thoughts about harming yourself

Serious allergic reaction

It is possible to have a serious allergic reaction (anaphylaxis) to montelukast.

Immediate action required: Call 999 now if:

  • your lips, mouth, throat or tongue suddenly become swollen
  • you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
  • your throat feels tight or you're struggling to swallow
  • your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
  • you suddenly become very confused, drowsy or dizzy
  • someone faints and cannot be woken up
  • a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)

You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.

These can be signs of a serious allergic reaction and may need immediate treatment in hospital.

Other side effects

These are not all the side effects of montelukast. For a full list see the leaflet inside your medicines packet.


You can report any suspected side effect using the Yellow Card safety scheme.

Visit Yellow Card for further information.

Page last reviewed: 14 March 2023
Next review due: 14 March 2026