L is licensed for the acute and long-term management of mania in bipolar affective disorder (electronic Medicines Compendium, 2014). Lithium has a narrow therapeutic index, and the National Institute for Health and Care Excellence (NICE, 2014) recommends regular therapeutic drug monitoring and that health professionals should aim to achieve a lithium plasma concentration between 0.6 and 0.8 mmol/litre in patients initially treated with lithium. For those already on lithium and who have relapsed, a plasma concentration between 0.8–1.0 mmol/litre is recommended. If a patient’s lithium plasma concentration is suboptimal, they are at risk of relapsing with either a manic or depressive episode. Concentrations above the therapeutic range can result in lithium toxicity. Each preparation of lithium differs in its pharmacokinetic properties and the amount of lithium contained, therefore it is important that lithium is prescribed by brand name (Joint Formulary Committee (JFC), 2015). In addition, when substituting different preparations, it is important to ensure the conversion is done correctly. A patient is diagnosed with bipolar affective disorder and is currently prescribed Priadel (lithium carbonate) tablets 600 mg at night as prophylaxis of manic episodes. He has relapsed and is currently secreting his tablets, and the prescriber wishes to convert him to a liquid preparation. Priadel tablets contain 5.4 mmol of lithium per 200 mg tablet. Priadel liquid (lithium citrate) has a strength of 520 mg per 5 ml, and each 5 ml contains approximately 5.4 mmol (JFC, 2015). Priadel 200 mg tablets cost £2.30 per pack (containing 100 tablets), and 400 mg tablets cost £3.35 per pack (also containing 100 tablets). Priadel liquid costs £5.61 per 150 ml bottle (JFC, 2015).