Antipsychotic drugs treat psychotic symptoms. These drugs are divided into two general categories, typicals and atypicals. Typicals date back to the early 1950s when chlorpromazine (Thorazine) was developed in a Parisian laboratory. These medications—which include such drugs as haloperidol (Haldol), thioridazine (Mellaril), and fluphenazine (Prolixin)—operate on the dopamine neurotransmitter system, specifically on the D₂ neurotransmitter receptors.

Typical antipsychotics are highly effective drugs, but have a range of problematic side effects. Anticholinergic effects, including dry mouth, blurred vision, and confusion, are found in low potency typicals, such as chlorpromazine and thioridazine. Extra-pyramidal side effects (EPS) are more common in high-potency typicals, such as haloperidol and fluphenazine. Symptoms of EPS include muscle tremors and stiffness.

Atypical antipsychotics include risperidone (Risperdal), olanzapine (Zyprexa), que-tiapine (Seroquel), ziprasidone (Geodon), and aripiprazole (Aricept). The atypicals entered the market in the 1990s, although the first atypical, clozapine (Clozaril), was introduced considerably earlier but fell out of favor due to the risk of agranulocytosis, a potentially fatal disorder of the white blood cells. Atypicals work on a range of neurotransmitters, including serotonin, dopamine, and norepinephrine. While atypicals are less likely to cause the kinds of side effects associated with typical antipsychotics, they bring their own side-effect profile. Most importantly, atypicals have a higher risk of metabolic syndrome, characterized by insulin resistance, high blood pressure, weight gain, and high blood sugar. Metabolic syndrome raises the risk of diabetes and heart disease. With the advent of atypicals, clozapine received new attention. It is now considered perhaps the most effective antipsychotic medication. Because of its side effect profile, however, it is only used after other drugs have failed.