Montelukast
Montelukast is a leukotriene receptor antagonist. It blocks the action of cysteinyl leukotrienes, inflammatory mediators that constrict the airways and drive swelling and mucus, so it damps the inflammatory side of asthma rather than relaxing muscle directly. It is taken as a tablet for asthma control and for allergic rhinitis, and is not a rescue treatment.
It works over days, so a reliever inhaler is still needed for sudden symptoms and the tablet is continued even when breathing feels fine. Food does not matter much, and it is usually taken in the evening, which fits the night-time and early-morning pattern of asthma symptoms. The reason for the mood warning is a recognised class effect on the nervous system: sleep disturbance, vivid dreams, irritability, anxiety, low mood and, rarely, thoughts of self-harm have been reported, in adults and children alike, sometimes soon after starting.
New or worsening low mood, agitation, unusual behaviour changes, or any thoughts of self-harm mean see a doctor without delay. Asthma that keeps worsening despite regular use, or rising reliever need, also needs review.
Reference information, not medical advice. Discuss any change with your doctor.