Hello,
Side effects of singulair are many varying from non serious ones like nausea,abdominal pain,headache,heart burns
and GI discomfort to serious ones like anaphylaxis,liver damage
and depression
and hallucinations.You can take zytec(cetrizine) for your allergies but it is also not without side effects.They may vary from allergic reactions to raised heart beat,seizures and liver damage.