How can the greatly higher anabolic effects of the long chain esters be explained?
While the authors do not make note of it in either article cited, there is a simple explanation for the observed result. Long chain esters of anabolic steroids are not many more times potent than short chain, if indeed they are any more potent at all. Yet in the above study, the undecanoate ester was found to give 3.5 times the effect of the propionate ester. Why?
There is a difference in pharmacokinetics (the time course of the drug in the body). Although the same 1 mg dose is being given in each case, it is either present in the serum of the animal at a relatively high concentration for a relatively short time for the shorter chain esters, or at lower concentration for a longer time for the longer chain esters. This difference can be quite large: the undecanoate ester can be predicted to have a half-life 36 times longer than that of the propionate ester.3
With most drugs, response is not proportional to the dose, but to the log of the dose. Assuming that the dose is well into the effective range, taking ¼ the dose does not result in only ¼ the result, but in ½ the result.
Viewed in this light, if the nandrolone propionate had been given in 36 divided doses over the same length of time that nandrolone undecanoate was in the system, in a manner to match its pharmacokinetics, one would expect 1/6 the result from each individual dose before accounting for molecular weight differences. The cumulative response would be 36 times 1/6, or six times the observed result from the single large dose. If we then correct for the lower molecular weight of the propionate ester, which delivers more nandrolone per mg. than does the undecanoate ester, we would predict 3.3 times more response than from the single large dose. In fact the observed response of the undecanoate ester was 3.5 times that of the propionate ester. This difference is within experimental error.
This calculation I have performed is also supported by experimental evidence performed by van der Vies4. His research showed that when the dose of nandrolone was divided into frequent small injections in such a pattern as to mimic the pharmacokinetics of esters, the anabolic effect became identical to that of the esters.Thus, pharmacokinetics, the log dose/response curve, and differences in molecular weight are sufficient to account for observed differences in anabolic effect between different esters of an anabolic steroid, or between an ester and the parent drug.
This correlates with my observation that anabolic effect of testosterone esters is equal, so long as each is administered reasonably frequently: at least once per half-life, and preferably twice. E.g., if testosterone propionate yielding some given amount of testosterone per week is administered daily, or at least every other day, it will give results comparable to testosterone cypionate administered at least once every week, and preferably twice per week, that yields the same amount of testosterone per week..
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