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Lithium is commonly used in treating psychiatric conditions such as bipolar disorder (manic depression). When used as a maintenance medication, lithium levels must be carefully controlled. Blood levels of lithium can be altered, if the patient is taking other drugs that react with the lithium. Diuretics, drugs that increase the secretion of urine, are thought to increase lithium retention. Studies have shown this occurs when sodium-depleting diuretics are taken, but other classes of diuretics have not been as well studied. A diuretic that does not cause this reaction is desirable, because patients taking lithium may also develop a disorder requiring diuretic treatment, such as high blood pressure. This study examined the effects of two diuretics from different classes, furosemide and hydrochlorothiazide, on lithium levels in 13 subjects. The subjects were healthy, male volunteers, who were given lithium for six weeks and either furosemide, hydrochlorothiazide, or a placebo for one week during weeks two, four, and six. Serum lithium levels were measured at the beginning of each week during which the other drugs were given, and then again on day five of the drug treatment. Serum lithium levels were significantly higher on day five compared with baseline levels when the subjects received hydrochlorothiazide. There were no significant changes when either the placebo or when furosemide was given. Diuretic activity, including sodium excretion, was significantly greater with hydrochlorothiazide than with either the placebo or furosemide. These results should be interpreted with caution; the subjects in this study were healthy volunteers, rather than patients, and therapeutic doses of lithium were not administered. However, it appears that furosemide may be a safe diuretic to prescribe in conjunction with lithium. (Consumer Summary produced by Reliance Medical Information, Inc.)