• you are allergic to any ingredient in Singulair.

Contact your doctor right away if any of these apply to you.

Some medical conditions may interact with Singulair. Tell your doctor or pharmacist if you have any medical conditions, especially if any of the following apply to you:

• if you are pregnant, planning to become pregnant, or are breast-feeding
• if you are taking any prescription or nonprescription medicine, herbal preparation, or dietary supplement
• if you have allergies to medicines, foods, or other substances
• if you have liver problems
• if you have a history of mental or mood problems, or suicidal thoughts or actions.

Some medicines may interact  with Singulair. Tell your health care provider if you are taking any other medicines, especially any of the following:

• Certain anticonvulsants (eg, phenobarbital) or rifampin because they may decrease Singulair's effectiveness.

This may not be a complete list of all interactions that may occur. Ask your health care provider if Singulair may interact with other medicines that you take. Check with your health care provider before you start, stop, or change the dose of any medicine.

Important safety information:

• Singulair may cause drowsiness or dizziness. These effects may be worse if you take it with alcohol or certain medicines. Use Singulair with caution. Do not drive or perform other possible unsafe tasks until you know how you react to it.
• Singulair will not stop an asthma attack once one has started. Be sure you always carry appropriate medicine (eg, bronchodilator inhalers) with you in case of an asthma attack.
• Contact your doctor promptly if your short-acting inhaler use increases or if use exceeds the 24-hour maximum prescribed by your doctor. Contact your doctor if your asthma worsens.
• Do not decrease your dose or stop using Singulair or other asthma medicines without first checking with your doctor.
• If your asthma is sensitive to aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs) (eg, ibuprofen), continue to avoid those medicines as directed by your doctor.
• Some patients taking Singulair have developed mental or mood changes, including suicidal thoughts or actions. Contact your doctor immediately if you experience symptoms such as agitation, aggression, hostility, anxiety, depression, strange dreams, trouble sleeping, sleepwalking, tremor, hallucinations, restlessness, irritability, or any unusual change in mood or behavior. Contact your doctor immediately if any signs of suicidal thoughts or actions occur.
• Pregnancy and breast-feeding: If you become pregnant, contact your doctor right away. You will need to discuss the benefits and risks of using Singulair while you are pregnant. It is not known if Singulair is found in breast milk. If you are or will be breast-feeding while you use Singulair, check with your doctor. Discuss any possible risks to your baby.

Check with your doctor if any of these most common side effects persist or become bothersome:

Cough; dizziness; headache; indigestion; nausea; stomach upset or pain; stuffy nose; tiredness; trouble sleeping; weakness.

Seek medical attention right away if any of these severe side effects occur:

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); aggressive behavior; agitation; chest pain; dark urine; fever; flu-like symptoms; hallucinations; irregular heartbeat; mental or mood changes; new or worsening wheezing or other breathing problems; numbness or tingling of the hands or feet; seizures; severe or persistent stomach pain; severe sinus inflammation; suicidal thoughts or actions; swelling; tremor; unusual bruising or bleeding; upper respiratory tract infection; yellowing of the skin or eyes.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider.