Those subjects in the present study who were chronically taking either tricyclic antidepressants or lithium reported a potentiation in the subjective effects of LSD [...] However, although acute and subchronic administration of lithium may increase serotonin levels in the brain, chronic administration of lithium has been shown to decrease 5-HT concentrations. This may be important since it has been suggested that LSD is a partial agonist, with its effects dependent on endogenous 5-HT concentrations. Since levels of 5-HT are reduced after chronic lithium, this may allow LSD to act as an agonist behaviorally and thus account for the data in the present investigation.