Tamoxifen (Nolvadex) is a drug that is not widely used for men's issues based on what I have seen in the Peak Testosterone Forum. I was getting a slight amount of gyno when I first started my weekly testosterone cypionate injections and was offered tamoxifen. (Letrozole is used more by the steroid guys from what I have read, but this is what my clinic offered.) I solved the problems, actually, by losing a little weight and dropping my cypionate dosage. I also had an interesting case on the Peak Testosterone Forum where one man was using it in place of Clomid to raise his testosterone, because he was concerned about Clomid's potential vision issues.  (See my link on Potential Risks of Clomid for more information.) And it reportedly raised his testosterone from 371 to 533 ng/dl, a modest increase at least, which is what any self-respecting SERM would do.
This forum poster also had mildly elevated prolactin and was concerned about a prolactinoma. Of course, prolactinomas actually secrete extra prolactin by definition and can elevate a man's prolactin levels substantially. When this occurs, it is very bad for the bedroom since the extra prolactin can lower testosterone and libido. And, of course, a large prolactinoma can cause other health issues, such as headaches if it grows too large. And this poster was, indeed, experiencing may of the standard symptoms that accompany elevated prolactin and low testosterone: "erectile dysfuction, fatigue, low motivation, difficulty concentrating, grumpy, barely any morning wood, bad sleep."
So was the tamoxifen he was taking raising or lowering his testosterone? Well, there is no great study on men, but what research we do have all seems to indicate the same thing: tamoxifen, in general, lowers prolactin:
1. Rats with Prolactinomas. One study implanted prolactin-secreting tumors into rats and found that tamoxifen actually decreased tumor size and lowered prolactin levels.  In fact, it did it better than the frontline drug bromocriptine. This is a remarkable result, but, as far as I know, tamoxifen is not used in the battle against prolactinomas by physicians here in the U.S. Cabergoline, bromocriptine and surgery seem to be the standard medical treatments and you can read more the subject in my link on Testosterone and Prolactin.
2. Male Rats with Elevated Prolactin. Another study looked at rats who had their prolactin levels artificially increased with estradiol, which also enlarged the pituitary itself.  Remarkably, tamoxifen blocked both of these, i.e. normalized pituitary weight and lowered prolactin levels and, once again, outperformed bromocriptine! As always, the problem is that this would be an off label use of tamoxifen and most physicians are unwilling to experiment with non-FDA approved usages of a medication.
3. Female Rats without Ovaries. More verification came from a study of female rats where researchers studied the increase in the uterus and prolactin levels from injected estradiol. The authors stated that "the results of the present paper showed that tamoxifen reduced estrogen-stimulated prolactin levels in some, but not in other hormonal conditions and that these effects were not mediated by an inhibition of lactotroph cell growth." In the female rats, bromocriptine was actually more powerful. Nevertheless, the tamoxifen did have a significant effect.
Of course, one should never self-treat when it comes to elevated prolactin levels or a prolactinoma. Both can be potentially crippling and, of course, affect one's sex life and marriage. So always work with your doctor.
Nevertheless, tamoxifen's effect on prolactin and related tumors is interesting, because in many men it would have a much better side effect profile. Both cabergoline and bromocriptine are notorious for rather harsh side effects. Tamoxifen would also have the nice advantage of often raising testosterone levels a little as well as it did for the man above.
CAUTION: Side effects are very high on higher dosage of tamoxifen used for cancer patients. See this study for an example.  In addition, long term effects are poorly understood. Discuss with your physician.
2) J Endocrinol, 1980 Jul, 86(1):109-16, "Tamoxifen suppresses both the growth of prolactin-secreting pituitary tumours and normal prolactin synthesis in the rat"
3) Cancer, 1984 Apr 1, 53(7):1473-7, "Comparative effects of tamoxifen and bromocriptine on prolactin and pituitary weight in estradiol-treated male rats"
4) Horm Metab Res. 1996 Apr, 28(4):171-6, "Effects of tamoxifen on serum prolactin levels, pituitary immunoreactive prolactin cells and uterine growth in estradiol-treated ovariectomized rats"
5) Annals of Oncology, "Retrospective review of male breast cancer patients: analysis of tamoxifen-related side-effects"