It's highly toxic, carcinogenic, and not very effective.
Nonsurgical treatment of male infertility: specific and empiric therapy
"Tamoxifen citrate is an antiestrogen that exhibits less estrogenic activity than clomiphene citrate and has been used in the treatment of male infertility. Doses range from 10 to 30 mg orally per day. Side effects are similar to those seen with clomiphene citrate but occur with lower frequency because of its weaker estrogenic properties. Although initial uncontrolled studies reported impressive results, including increased sperm densities and pregnancy rates (Vermeulen and Comhaire 1978; Bartsch and Scheiber 1981; Buvat et al 1983),
all controlled studies using tamoxifen 10 to 20 mg per day reported negative results (Willis et al 1977; AinMelk et al 1987; Krause et al 1992)."
There are other threads with supporting studies on the toxicity of it.
The alternative is Clomid or Torem