Nolvadex (tamoxifen citrate) is very comparable to Clomid, behaves in the same manner in all tissues, and is a mixed estrogen agonist/antagonist of the same type as Clomid. The two molecules are also very similar in structure. It is not correct that Nolvadex reduces levels of estrogen: rather, it blocks estrogen from estrogen receptors and, in those tissues where it is an antagonist, causes the receptor to do nothing. Nolvadex is not an anabolic steroid but is used to manage side effects from steroids.
The claim that Nolvadex reduces gains should not be taken too seriously. The fact is that any number of bodybuilders have made excellent gains while using Nolvadex. The belief that it reduces gains seems to stem from the fact that the scientific literature reports a slight reduction in IGF-1 (individuals using anabolic steroids were not studied though) from use of Nolvadex. Thus, Dan Duchaine reported that it reduces IGF-1 and therefore reduces gains. However, if this effect exists at all, it must be very minor, due to the excellent gains that many have made, and from the fact that no one has noticed any such thing from Clomid, which has the same activity profile.
However, I would not be surprised if one were to tell a steroid user that Clomid reduced his gains, he would immediately become afraid that Clomid reduced his gains (please note that no one I have ever heard of has noticed this.) Not having been so misled, however, he would not conclude this from his results. But if an authority publishes that such an effect occurs, whether it does or not it can become self-fulfilling by biasing the user.
The fact that Nolvadex will reduce water retention may result in the user agreeing that gains are less, since weight gain is less, thus reinforcing the bias.
Tamoxifen citrate is the chemical name of active ingredient in Nolvadex. Nolvadex is a registered trademark of AstraZeneca UK Limited in the United States and/or other countries.