Singulair MUST be taken in the evening for asthma.
Asthma symptoms tend to be worse during the night and early morning hours. Singulair (montelukast) has a short half-life of about 4-7 hours with blood levels peaking 3-4 hours after a dose.
The prescribing information for Singulair recommends taking it at night in order to match peak drug levels with symptom onset.
But there's no proof that nighttime dosing is better than morning dosing. None of the clinical trials that studied Singulair for asthma looked at morning dosing.