The renal clearance of lithium will decrease, and hence the risk of acute lithium toxicity will increase, in any situation leading to dehydration and sodium depletion. Patients on long term lithium therapy with progressively declining urinary concentrating ability may be at special risk in this regard.Chronic histological changes in the kidney attributed to lithium therapy correlate with age rather than with the duration of lithium therapy. Age-related renal histological changes are associated with decreased glomerular filtration rate and therefore reduced renal lithium clearance. Thus, the dose of lithium should be reduced with advancing age.