A: Zyprexa (olanzapine) is an antipsychotic medication used in the treatment of various psychoses and behavioral or mood disorders. The most common side effects with Zyprexa are drowsiness, weight gain, nausea, dry mouth, constipation, and dizziness.Other side effects of Zyprexa include hyperglycemia and diabetes.yprexa is part of a class of drugs known as atypical antipsychotics, which work to help restore the balance of dopamine, a natural substances in your brain known as a neurotransmitter. By taking this drug you may be able to decrease hallucinations, reduce feelings of agitation, and think more clearly and positively.