This article is from the Rat Health Care booklet. In several experiments, tamoxifen has been found to be effective in protecting rats against mammary cancers which have been chemically induced. Unfortunately I found only one study showing the effect of tamoxifen on spontaneous tumors. So what researchers are interested in is the effect of tamoxifen on breast cancer.
Tamoxifen seems to be a very interesting drug with different effects in different species.
For instance, in chicks it has a strictly anti-estrogen effect, while in dogs it has a mainly estrogen effect. For this reason, tamoxifen does not seem to be an effective treatment for mammary tumors in dogs. In one study, about half the spayed dogs given tamoxifen had side effects caused by the estrogen effect and it has strongly negative effects on intact female dogs, causing pyometra, a serious infection of the uterus. In humans and rats, tamoxifen has both anti-estrogen and estrogen effects on different tissues. Scientists don't know why the effect is different in different species.