Some bodyweight gain is common with many of the psychotropic drugs frequently used to treat psychiatric disorders, Although in most instances the gain is not clinically significant, that induced by some drugs, such as lithium and atypical antipsychotics, can be quite large and clinically significant. Individual patients may not wish to continue with medications that increase bodyweight, even by amounts that have no significant effect on health, The true prevalence of bodyweight gain is often difficult to discern because of confounding factors within studies that have assessed this effect, such as the simultaneous use of multiple drugs and the absence of data on starting bodyweight. Risk factors for weight increase have not yet been well characterised, although in general the potential for drug-induced bodyweight gain is greatest in individuals with a past and/or family history of obesity. Little is currently known about the mechanisms of bodyweight gain that is induced by any drug.