Zyprexa (olanzapine)
Atypical antipsychotics, such as Zyprexa and Clozaril (clozapine), can pile on the pounds. A 2005 study found that 30% of people on Zyprexa gained 7% or more of their body weight within 18 months.
These drugs, used for schizophrenia and bipolar disorder, have potent antihistamine activity and inhibit serotonin, which may trigger weight gain.