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What is Long-term tamoxifen therapy may cause: and explain in detail?
Long-term tamoxifen therapy is a predisposing factor for endometrial hyperplasia and cancers.
Malignancies caused by Long-term tamoxifen therapy:

Carcinoma endometrium-it is the most common carcinoma associated with it.
Uterine sarcoma.
Rarely liver cancer with a long-term high dose.

Non-malignant effects of tamoxifen on uterus:

Endometrial hyperplasia
Endometriosis
Fibroid uterus
Ovarian cysts
Menstrual irregularities or amenorrhea.