Side effects are unwanted responses to a medication when it is taken in normal doses, which can range from mild to severe. As with all medications, SINGULAIR also has some side effects which may not affect all patients. Many of these side effects can be managed, while some may go away on their own over time. Speak to your physician right away if you have any of these symptoms while taking SINGULAIR: agitation including aggressive behavior or hostility, bad or vivid dreams, depression, disorientation/confusion, anxiety, hallucination, irritability or restlessness, sleepwalking, suicidal thoughts and actions, tremors, trouble sleeping, feeling of pins and needles or numbness of arms or legs, a flulike illness, rash, severe inflammation of the sinuses, irregular heartbeat upper respiratory infection, fever, headache, sore throat, cough, stomach pain, diarrhea, earache or ear infection, flu, runny nose, sinus infection increased bleeding tendency, allergic reactions (including swelling of the face, lips, tongue, and/or throat [which may cause trouble breathing or swallowing], hives, and itching), dizziness, drowsiness, pins and needles/numbness, seizures, palpitations, nosebleed, stuffy nose, diarrhea, heartburn, indigestion, inflammation of the pancreas, nausea, stomach or intestinal discomfort, vomiting, hepatitis, bruising, rash, joint pain, muscle aches, and muscle cramps, tiredness, swelling