Studies on the metabolism and pharmacokinetics of tamoxifen in normal volunteers.

The densitometric analytic procedure used for tamoxifen can also be used to quantify its desmethyl metabolite. In a study involving six healthy male volunteers, tamoxifen tablets were shown to be as bioavailable as a solution of tamoxifen citrate. After administration of a single dose of 20 mg, peak serum levels of tamoxifen were 42 ng/ml; those of the metabolite were 12 ng/ml. The half-lives of the drug and metabolite were approximately 4 and 9 days, respectively, after a single dose. After three widely separated single doses, a reversible increase in elimination half-life occurred.