Pharmacokinetics of anabolic steroids

After an anabolic steroid is administered orally there is a rapid increase in its concentration in the blood in the following few hours. Excretion of the compound and its metabolites takes place via the urine and faeces, usually taking several days to completely pass through the system. Parenteral anabolic preparations, such as microcrystal suspensions, implants and solutions of nandrolone esters in a vegetable oil, are absorbed slowly. The absorption of esters from the site of injection is a logarithmic process and depends on the nature of the ester concerned. The absorption rate is slower the longer the fatty acid chain in the ester. The rate of absorption is important regarding the length of the action, and is also relevant for the compound's pharmacodynamic pattern. Metabolism of anabolic steroids takes place mainly in the liver and involves reduction, hydroxylation and the formation of conjugates. The enzymes that bring about these changes and the metabolic pathways involved are similar to endogenous steroids. The presence of high concentrations of metabolites in the bile and in the intestinal tract points to enterohepatic circulation of metabolites. After the administration of slowly-absorbed nandrolone esters the excretion of the metabolites continues for weeks or even months. It is probable that the absorption rate of the esters from the injection depot is the rate-limiting process in the pharmacokinetics of these compounds.