Dosage and administration
Initiation is usually by a specialist and the patient will normally be stabilised by the time they reach care in general practice. The GP has a vital role in maintenance of optimal treatment and identifying the need for dosage adjustment. The daily dose is individualised according to therapeutic response and serum drug concentrations but is usually in the range 400-1200 mg daily. Lithium may be given as a single daily dose in the morning or at bedtime. Alternatively, the dose may be divided and given morning and evening, however polyuria can occur more frequently with twice daily dosing. Once-daily dosing may reduce the risk of renal damage and adverse effects and simplifies the dosing regimen.3,4 For these reasons once-daily dosing, may be preferred.
Dosage adjustment
Lithium exhibits linear pharmacokinetics, which means that the steady state serum concentration is proportional to the dose, therefore doubling the dose will double the concentration and vice-versa (see example below).
Formulations of lithium
Lithium is available in New Zealand as lithium carbonate. There are three different forms of lithium available:
- Lithium Carbonate 250 mg capsules (Douglas brand).
- Lithium Carbonate 250 mg and 400 mg film coated tablets (Lithicarb).
- Lithium Carbonate 400 mg slow release tablets (Priadel).
These forms have different bioavailability and different release profiles. For this reason these are not interchangeable. When prescribing lithium it is important that you specify which brand or form is to be dispensed.
If it is necessary to change brands of lithium, additional lithium serum level monitoring should be instituted, until the level has stabilised. The level should be measured five days after the change, then weekly until restabilised at the desired serum level.
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| Dose adjustment example: A person’s serum concentration is 1.0 mmol/L after a week of reliable dosing on a daily dose of 800 mg lithium carbonate. A reduction in dose to 600 mg should give a new steady state concentration of 0.75 mmol/L. This should be checked at least five days after the dose reduction. |
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