Pharmacokinetic interactions involving psychoactive agents are better quantified than those implicating pharmacodynamic mechanisms, though often only in healthy subjects. Most of these drugs display a clear concentration effect relationship. Thus, a change in plasma steady state concentration will probably be reflected in an altered response. The extent and effect of these interactions will vary between individuals but is likely that adverse consequences can be anticipated in some cases. Many drugs having a similar chemical structure share the same pharmacological properties and may be expected to be implicated in similar adverse interactions. The important and better documented adverse drug interactions are listed.