The pharmacokinetics of depot antipsychotic medications are such that an intramuscular injection given at intervals of from 1 to 4 weeks will produce adequate plasma concentrations that are sufficient to prevent relapse over the dosage interval. Such medication is useful in patients who do not reliably take their oral medication. The pharmacokinetics and clinical actions of various depot formulations of antipsychotic drugs have been extensively studied. Unfortunately, patients who do not reliably take their oral medications are unlikely to volunteer for controlled studies. This is because the same factors that influence a patient to not cooperate with the physician in taking the medication as prescribed will also interfere with their willingness to volunteer for research protocols. Thus, evidence from blinded controlled trials may not necessarily reflect the actual patient population at risk. We feel that particularly important evidence of efficacy of depot vs oral medication comes from mirror-image studies. In these trials, the number of hospitalisations after initiation of depot medication is compared with that observed when the patient was solely taking oral medication. Studies of this type show that depot medication substantially reduces the rate of relapse. There is considerable evidence about how long depot medications should be used. For many patients, depot medication to prevent relapse in schizophrenia should be used for the life of the patient. As the conventional antipsychotic agents are replaced by a new generation of agents, the need for depot formulations will continue, and the knowledge gained about the current formulations should transfer to future generations of drugs.