Why would my doctor tell me to avoid tamoxifen?

Risks vs. benefits. Tamoxifen is a very effective drug for the prevention and treatment of hormone-sensitive breast cancer. However, like all medications, there are small risks associated with tamoxifen, including a risk of blood clots, stroke, uterine cancer, and cataracts. I would suspect that your dr. Felt that the risks outweighed the benefits in your case or that your cancer was not sensitive to tamoxifen.
Speak to you doctor. I recommend speaking with you doctor. Do you have breast cancer? It would be a good idea to get a 2nd opinion.