Does nolvadex (tamoxifen) decrease testosterone production in males?

No: Nolvadex, a brand of tamoxifen, is a selective estrogen receptor modulator. As such, there are some small studies that suggest that it can actually increase testosterone production in med. Similarly, so can aromatase inhibitors which decrease conversion of t into e. Check out http://en.Wikipedia.Org/wiki/list_of_drugs_banned_from_the_olympics for a list of banned substances in competition & why.