The therapeutic dosage of lithium is close to the toxic level, and drugs reducing the renal clearance of lithium can produce the most serious adverse effects. When used in combination with psychiatric drugs affecting mood, the therapeutic potential of lithium may be enhanced, and earlier concern regarding possible neurotoxic interactions with neuroleptics appear poorly substantiated. A number of specific interactions of low incidence have also been reported which should be borne in mind when new medications are introduced. The ready availability of lithium concentration monitoring in body fluids makes it possible to alter the effects of some potentially interactive drugs that may still need to be given in combination with lithium.