Hi, Thanks for posting in HCM. i understand your concern. Singulair contains montelukast as ingredient, which is a leukotriene inhibitor. It is used to prevent asthma attacks and treat allergies in adults and children. You need to seek medical attention promptly if you experience any of the following side effects: 1. mental/mood changes such as agitation, aggression, anxiety, sleeplessness, abnormal dreams, sleep-walking, memory/attention problems, depression, hallucinations, thoughts of harming yourself/suicide 2. numbness/tingling/shooting pain in the arms or legs 3. sinus pain/swelling 4. muscle weakness Breast symptoms what you are experiencing might not be associated with intake of the medication. Kindly consult Physician in this regard to know what could be the cause, so that it can be treated accordingly. Hope the information provided would be helpful. All the best.