This information is for parents of a child who has asthma. It tells you about leukotriene antagonist tablets, a treatment used to prevent asthma symptoms. It is based on the best and most up-to-date research.
Do they work?
Yes. A leukotriene antagonist tablet called montelukast (brand name Singulair) can help to prevent asthma symptoms in children over 2 years old.
But these tablets may not work quite as well as steroids from an inhaler, the main treatment to prevent asthma attacks in children.
What are they?
Leukotrienes are natural chemicals your child's body makes when he or she comes into contact with an asthma trigger (such as house dust mite droppings). It is these chemicals that make the muscles in the airways tighten and make your child's breathing more difficult.
Leukotrienes also help the lungs produce mucus, making your child cough and the airways in the lungs inflamed and swollen.
Leukotriene antagonists are drugs that prevent asthma symptoms by stopping leukotrienes from making the airways in your child's lungs inflamed.
There are two leukotriene antagonists: zafirlukast (brand name Accolate) and montelukast (brand name Singulair). Both come as tablets. Zafirlukast is not recommended for children under 12 years old. All the research in children looks at montelukast.
Your doctor or nurse may refer to this medicine as a preventer. These drugs are designed to be used together with other asthma treatments such as quick-relief inhalers ( salbutamol or terbutaline) and sometimes steroids from an inhaler.
How can they help?
If your child takes leukotriene antagonist tablets (the only one studied in children is montelukast), this is how they might help:    
Your child may get fewer symptoms during the day and at night
Your child may get better scores on lung tests
Your child may not need to use their quick-relief inhaler (salbutamol or terbutaline) so often
Montelukast doesn't seem to work as well as steriods from an inhaler.     
How do they work?
Children who have asthma have inflamed air passages. Leukotriene antagonists help because they open up the airways by reducing the swelling. They also help prevent further inflammation. This makes it easier for your child to breathe, and reduces wheezing.
They do their job by preventing leukotrienes from doing what they normally do. Leukotrienes are released into the blood by your child's immune system after he or she breathes in an asthma trigger (such as house dust mites or smoke). Leukotrienes are one of many chemicals that make the air passages swell up and breathing more difficult in children with asthma.
If the airways are less inflamed, breathing becomes easier.
Leukotriene antagonists also help to reduce the amount of sticky fluid (excess mucus) that can build up in air passages and block them.