Age as a factor affecting lithium therapy.

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Abstract
1 We examined the case notes of 82 psychiatric out‐patients (aged 21‐84 years) receiving lithium prophylaxis and with steady‐state plasma lithium levels. 2 The mean weight‐related daily dose of lithium prescribed decreased by about 50% between the third and eight decades. 3 The corresponding steady‐state plasma lithium levels showed a less marked tendency to decrease, this only being seen in the seventh and eighth decades. 4 In patients aged 50 years or over the daily lithium dose required to give a plasma level of 1 mmol l‐1 (0.50 mmol kg‐1) was significantly lower than that (0.65 mmol kg‐1) in patients aged under 50 years (P less than 05, Student's t‐test). In patients aged 70‐79 years this dose was 31% lower than in patients under 50 years. However, interindividual variation was great and it was estimated that age only contributed about 14% to the total interpatient variation. 5 Of the 36 patients under 50 years of age, 42% had minor lithium side‐effects and 17% were not optimally controlled with lithium. The corresponding figures for the 46 ‘older’ patients were 46% and 28%. 6 Generally the 50% dosage reduction seemed necessary to compensate for an age‐related decrease in lithium excretion and to reduce lithium side effects to a level comparable to that acceptable in younger patients.

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