Singulair (montelukast) will be in your system for approximately 30.3 hours.
The mean plasma half-life of montelukast ranges from 2.7 to 5.5 hours in healthy young adults. This is the time it takes for your body to reduce the plasma levels by half.
It takes approximately 5.5 x half-life for a drug to be out of your system therefore Singulair will be in your system for approximately 5.5 x 5.5 hours = 30.3 hours

Other factors also need to be taken into account and include:
1. How much and how often you have taken the drug.
2. Your metabolic rate – a slower metabolism will increase the time a drug remains in your system.
3. Your age and health – older age and poor health will generally increase the time the drug stays in your system.
4. Body mass – generally the bigger you are the longer a drug will remain in your system.

Singulair (montelukast) belongs to a group of medications known as leukotriene receptor antagonists. It is used to prevent and manage asthma symptoms and to relieve the symptoms of seasonal allergies, also known as seasonal allergic rhinitis or hay fever.

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