There are no unpleasant side effects for me, and Tamoxifen is an anti-estrogen, actually that's not quite true (it seems to more selective). You are correct, though, no medicine works the same for everyone.
"While Tamoxifen is commonly known as an anti-estrogen, this is not an accurate description of its clinical activity. In actual fact, Tamoxifen boasts both estrogenic and anti-estrogenic properties depending on the target tissue. Due to this dual action, Tamoxifen is better described with the term selective estrogen receptor modulator (SERM)." The effects on the prostate, over all, seem to be beneficial.
If you happen to have moderate to severe BPH, you know that any relief is welcome. While, usually not fatal in and of itself, it does lower your quality of life. I'm in the Philippines and can get just about anything OTC. The tamoxifen is branded and reasonably priced. Presently not taking Cialis, but could still get reasonably priced Cialis from Cipla, India.
The selective action on the prostate is quite exciting, especially with the potential ability to shrink the gland and lower PSA (an indication that it is having a beneficial affect). It wouldn't surprise me if researchers find it lowers the risk of prostate cancer, as well as other types, just like it does breast cancer. The financial incentive to conduct more research is likely limited because it's a relatively inexpensive medicine. Sorry, there's a bit of cynisism here due to what pharmacutical companies do in America (my home county of origin).