Medifit Biologicals suggests that, in order to design an effective steroid cycle, one must have a sound understanding of steroid pharmacokinetics.
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.
From a kinetic viewpoint, the rate of appearance of steroid in the blood following an intramuscular depot injection is governed by several simultaneous processes. If one were to measure the levels of steroid in the blood as a function of time, what is typically observed is a rapid increase in concentration (characterized by an initial steep ascending phase of the concentration versus time curve) followed by a relatively slow decrease in concentration (characterized by a relatively shallow descending phase of the concentration versus time curve). The former process reflects the combined processes of hydrolysis of the ester and of its subsequent distribution and elimination whereas the latter process represents the process of ester release (absorption) from the injection depot into the bloodstream.
Thus, the overall behavior of a steroid in the blood (its rate of appearance and subsequent disappearance) is dependent upon several simultaneous processes, all of which display differing half–lives, making mathematical deconvolution of each individual process kinetically difficult. However, a characteristic of depot formulations is that the process of steroid release from the injection depot to the general circulation is the slowest process of all, making it the rate–limiting step in the sequential/parallel processes of absorption, distribution and elimination. For this reason, the situation is now simplified because the plasma concentration–time profile tends to closely parallel rate of absorption. Put another way, the half–life of a steroid, when administered as a depot preparation, is a reflection of the rate and extent of absorption and not elimination or distribution. It therefore follows that by simply having some knowledge a steroid’s disposition half–life, we can, to a very good approximation, predict how the concentration of steroid in our body changes with time.