KINETICS

Absorption by route of exposure The absorption after oral dosing is rapid for testosterone and probably for other anabolic steroids, but there is extensive first-pass hepatic metabolism for all anabolic steroids except those that are substituted at the 17-alpha position.

The rate of absorption from subcutaneous or intramuscular depots depends on the product and its formulation. Absorption is slow for the lipid-soluble esters such as the cypionate or enanthate, and for oily suspensions.

Distribution by route of exposure The anabolic steroids are highly protein bound, and is carried in plasma by a specific protein called sex-hormone binding globulin.

Biological half-life by route of exposure The metabolism of absorbed drug is rapid, and the elimination half-life from plasma is very short. The duration of the biological effects is therefore determined almost entirely by the rate of absorption from subcutaneous or intramuscular depots, and on the de-esterification which precedes it (Wilson, 1992).

Metabolism Free (de-esterified) anabolic androgens are metabolized by hepatic mixed function oxidases (Wilson, 1992).

Elimination by route of exposure After administration of radiolabelled testosterone, about 90% of the radioactivity appears in the urine, and 6% in the faeces; there is some enterohepatic recirculation (Wilson, 1992).