This drug is unique (so far as I know) in that 5a -reductase, the enzyme which converts testosterone to the more-potent DHT, actually converts nandrolone to a less-potent compound. Therefore this AAS is somewhat deactivated in the skin, scalp, and prostate, and these tissues experience an effectively-lower androgen level than the rest of the body. Therefore, for the same amount of activity as another drug at the androgen receptors (ARs) in muscle tissue, Deca gives less activity in the scalp, skin, and prostate. Thus, it is the best choice for those particularly concerned with these things.
Its effectiveness at the androgen receptor of muscle tissue is superior to that of testosterone: it binds better. Yet, it gives only about half the muscle-building results per milligram. This I think is a result of its being less effective or entirely ineffective in non-AR-mediated mechanisms for muscle growth.
It also appears less effective or entirely ineffective in activity on nerve cells, certainly on the nerve cells responsible for erectile function. Use of Deca as the sole AAS often results in complete inability to perform sexually.
These problems can be solved by combining with a drug that does supply the missing activity: e.g. testosterone.
Nandrolone is proven to be a progestin. This fact is of clear importance in bodybuilding, because while moderate Deca-only use actually lowers estrogen levels as a consequence of reducing natural testosterone levels and thus allowing the aromatase enzyme less substrate to work with, Deca nonetheless can cause gyno in some individuals. Furthermore, just as progesterone will to a point increase sex drive in women, and then often decrease it as levels get too high, high levels of progestogenic steroids can kill sex drive in male bodybuilders, though there is a great deal of individual variability as to what is too much.
Incidentally, this progestogenic activity also inhibits LH production, and contrary to common belief, even small amounts of Deca are quite inhibitory, approximately as much so as the same amount of testosterone.
To some extent, nandrolone aromatizes to estrogen, and it does not appear that this can be entirely blocked by use of aromatase inhibitors – indeed, aromatase may not be involved at all in this process (there is no evidence in humans that such occurs) with the enzyme CYP 2C11 being in my opinion the more likely candidate for this activity. In any case, Cytadren, an aromatase inhibitor, has not been found effective in avoiding aromatization of nandrolone.
The drug is moderately effective at doses of 400 mg/week. The long half-life of Deca-Durabolin makes it unsuited to short alternating cycles, but suitable for more traditional cycles, with a built-in self-tapering effect in the weeks following the last injection.