Montelukast may cause serious or life-threatening mental health changes while you are taking this medication or after treatment has stopped. Tell your doctor if you have or have ever had any type of mental illness. However, you should know that it is possible to develop these changes in mental health and behavior even if you have never had any mental health problems in the past. You should call your doctor right away and stop taking montelukast if you experience any of the following symptoms: agitation, aggressive behavior, anxiety, irritability, difficulty paying attention, memory loss or forgetfulness, confusion, unusual dreams, hallucinations (seeing things or hearing voices that do not exist), repeating thoughts that you cannot control, depression, difficulty falling asleep or staying asleep, restlessness, sleep walking, suicidal thoughts or actions (thinking about harming or killing yourself or planning or trying to do so), or tremor (uncontrollable shaking of a part of the body). Be sure that your family or caregiver knows which symptoms may be serious so they can call the doctor if you are unable to seek treatment on your own.