Find out how montelukast works as a treatment for asthma, how to take it, and what side effects to look out for.

What is montelukast?

Montelukast is one of a group of medicines called leukotriene receptor antagonists. It is also known by the brand name Singulair.

Your GP or asthma nurse may prescribe montelukast if you’re still getting asthma symptoms even though you’ve been taking your preventer inhaler every day as prescribed.

Montelukast is an add-on treatment, which means you take it alongside your preventer inhaler.

Use your reliever inhaler to treat an asthma attack

Do not use montelukast to treat an asthma attack. Always use your reliever inhaler

How does montelukast help asthma?

Asthma can cause your airways to swell and become narrow. Montelukast blocks one of the chemicals that is released when you come into contact with asthma triggers.  This helps stop your airways from swelling.  

If you take montelukast as prescribed it can help your asthma by:

  • making it easier for you to breathe 
  • preventing asthma attacks 
  • reducing the need for your reliever inhaler 
  • stopping your airways narrowing when you exercise
  • improving symptoms of seasonal allergies. 

Can I take montelukast?

Most adults and children aged at least six months old can take montelukast.

Before taking montelukast, tell your GP if you: 

  • have had an allergic reaction to any medicine
  • are pregnant
  • are breastfeeding
  • are trying to get pregnant
  • have a rare hereditary problem of galactose intolerance.

How do I take montelukast?

Always follow your doctor’s instructions about how and when to take montelukast. Most people take montelukast once a day in the evening. If exercise makes your asthma worse, your doctor might tell you to take montelukast two hours before you exercise.

Montelukast comes as a non-chewable tablet, chewable tablet, or granules in sachets.

  • Non-chewable tablets are for adults and young people aged 15 and over. 
  • Chewable tablets come in two doses, for children aged 6 to 14 and for children aged 2 to 5.  You or your child should take chewable tablets at least one hour before food, or two hours after food.  
  • Granules are for young children who cannot take chewable tablets. You can put them directly on your child’s tongue or mix them with a spoonful of cold, soft food such as yoghurt. Do not dissolve them in a drink. 

Keep taking your preventer inhaler

You must keep taking your preventer inhaler when you are taking montelukast. If you stop taking your preventer inhaler, your asthma symptoms could get worse.

If you miss a dose

If you miss a dose of montelukast, take it the next day at the usual time. Do not take two doses to make up for the one you forgot. 

If you take too much montelukast

Go to or call 111. Call 111 if you're asking about a child under the age of 5 years.

Side effects of montelukast

Like all medicines, montelukast can cause side effects in some people. The patient information leaflet inside your medicine packet will list all the possible side effects.

Common side effects

These side effects happen in more than 1 in 100 people.

  • diarrhoea
  • high temperature
  • headaches
  • stomach pain
  • feeling or being sick
  • a mild rash.

The NHS website has advice on how to manage these side effects.

Serious side effects

Some rare or uncommon side effects of montelukast can be more serious.  If you get one of these side effects, contact 111.

Contact 111 immediately if:

  • Your mood changes and you feel depressed or aggressive 
  • you're thinking of harming yourself 
  • you see things which are not there (hallucinations)
  • you're finding it harder than usual to concentrate or remember things
  • 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 have more than one of the following symptoms:
    • an illness that feels like flu
    • pins and needle
    • numb arms or legs
    • worsening of asthma symptoms
    • 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.

Go to or call 111. Call 111 if you're asking about a child under the age of 5 years.

Serious allergic reaction 

Serious allergic reactions to montelukast are rare, but if you get symptoms, it’s important to call 999 immediately.

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 are 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).

Get support

Call our Helpline for support with your condition. Get advice on your medicines, symptoms or travelling with a lung condition, or just call us to say hello.

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