For a general discussion of antipsychotic medications, such as those used often in the treatment of schizophrenia or bipolar disorder, see the Wikipedia entry.
The most important class of enzymes for the metabolism of most drugs are the cytochrome P450s. In general, a patient who is a poor metabolizer for a CYP known to metabolize a given drug may be at higher risk for adverse side effects from that drug, and may derive benefit from being prescribed lower doses of that drug, or alternatively, may benefit more from being prescribed an alternative medication metabolized by a different CYP for which that patient is a normal metabolizer.
For antipsychotic drugs, an overview (adapted from Wall CA et al., Prim. Psychiatry 17(5), 2010) of which CYP is the primary metabolizer (shown as 1), or a secondary/substantial metabolizer (2), or is quite minimally involved (0), is shown in the following table: