According to Google: “Tamoxifen was approved by the Food and Drug Administration (FDA) in the United States on December 31, 1977, for the treatment of advanced breast cancer. It was subsequently approved for the prevention of breast cancer in high-risk women in 1998. ...The third-generation aromatase inhibitors (letrozole, anastrozole, and exemestane) were approved in the late 1990s for estrogen-dependent postmenopausal breast cancer.”

I assume that the scientific studies that led to approval were done before 1998—in other words, almost 30 years ago. Perhaps your public library’s reference desk can help you search scientific journals for the studies that led to approval.

As best I can tell, prescribing AIs is considered “standard of care”. I’m guessing that by today’s standards it would be unethical to have a research study where one group got standard of care and another group got nothing. This video is 5+ years old, but I've never found anything better. It discusses how much each treatment (surgery, radiotherapy, AIs) reduces risk of recurrence.

I’m neither an MD nor a scientist and I have no connection with Oster Oncology. If anything I’ve said is incorrect, please let me know.