Singulair is a prescription medicine approved to help control asthma in adults and children as young as 12 months and to help relieve the symptoms of seasonal allergies in adults and children as young as 2 years. The reason why it works with both conditions is because the mediators present in asthma are also present in allergic rhinitis. In all of its studies, Singulair has only been tested with a single dose in the evening, so that is how it has been approved for use (it was originally formulated for relief of nighttime asthma symptoms). However, it is a once-a-day medication so its effects should last twenty-four hours. Singulair has no side effects such as drowsiness or dryness, like antihistamines. Singulair is a leukotriene antagonist, different than antihistamines.