The effect of age on lithium dosage requirements.

Analysis of more than 350 observations of patients who had been under lithium treatment for 6 months to 4 years showed a significant negative correlation between dosage requirements and age; old patients required less lithium than young patients to maintain a particular serum lithium concentration. The decline in dosage requirement with increasing age could be fully accounted for by the age-dependent fall in the glomerular filtration rate and lithium clearance. There was no age-dependent change in tablet compliance, and the duration of exposure to lithium did not influence the dosage requirement.