Anabolic/androgenic steroids may alter sexual desire and functioning. The nature of their effects, however, can vary depending on the drug(s) and dosage(s) used, as well as the individual sensitivity of the user to hormonal manipulation. One of the most common responses is a stimulatory one. Testosterone is the primary male sex steroid. As such, it is responsible for increasing sexual desire and supporting many male reproductive-system functions. Since all anabolic/androgenic steroids are derived from (and share similar action to) testosterone, use of these drugs is often linked to increases in sexual desire, as well as copulation and orgasm frequency. A significant increase in the frequency and duration of erections may also be noted. In most cases these side effects are not troubling, and may even be regarded as positive by the individual (some feel it improves their sex lives).
AAS use may also be associated with diminished libido and sexual functioning. This could be due to several factors. One is insufficient androgenic activity. This sometimes is noted with the use of primarily anabolic steroids such as methenolone, nandrolone, or oxandrolone, which may not provide sufficient androgenicity to compensate for suppressed endogenous testosterone. Studies also show that estrogen plays an important role in libido and sexual function in men. Therefore, the use of non-aromatizable steroids or aromatase-inhibiting drugs sometimes also causes interference. The addition or substitution of testosterone during a cycle is usually regarded as the most reliable way to correct issues with male sexual functioning, as it supplements the full spectrum of sex steroid activity. Note that sexual issues are also common after steroid discontinuance, when endogenous steroid levels are low.
Wlliam Llewellyn (2011) - Anabolics