It is also not unusual for bodybuilders to feel poorly on beginning a cycle of high dose testosterone without antiestrogens, and for this reason many have advocated starting with a low dose and building up. However, I strongly suspect that the real problem is estrogenic effect on mood, and the problem can be avoided with use of an aromatase inhibitor. Though most bodybuilders feel they know which steroids aromatize and which do not, sometimes the beliefs are in error. Both hormones can cause bloating, and both can cause gyno. So AAS which are capable of activating not only the androgen receptor but also the progesterone receptor are often mistakenly assumed to aromatize.
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. Well, antiestrogens can do nothing about these side effects of Deca. However the latter is usually used in doses low enough that aromatization is not an issue.