- It has been proposed that lithium acts by inhibiting inositol monophosphatase, thereby lowering tissue inositol levels. We therefore studied inositol effects on a prominent lithium side-effect, polyuria—polydipsia, in rats and in humans. Rats were treated with daily injections ip LiCl for 14 days and were given 2.5 per cent inositol, 2.5 per cent mannitol or no additive in their drinking water. Fourteen patients complaining of lithium side-effects were recruited for the study after approval by the Helsinki Committee. Patients were treated with inositol in 500 mg capsules, two capsules three times daily for 5 days. In both experiments inositol ameliorated Li-induced polyuria—polydipsia. However, mannitol had similar but less powerful and perhaps less lasting effects in rats.