But taxol isn't tamoxifen.

Here's the story of tamoxifen, an extract from an article by Cancer Research UK:

The story of tamoxifen starts back in 1896, when pioneering cancer surgeon Dr George Beatson found that he could extend the lives of women with breast cancer by surgically removing their ovaries – a major oestrogen source. This gave researchers the first clue that oestrogen was involved in the growth and development of breast cancer.

Over the next five decades, doctors experimented with a variety of man-made oestrogen-like chemicals (such as stilboestrol, developed by Cancer Research UK scientist Professor Charles Dodds in 1937) to try to treat breast cancer. Although sometimes these efforts were successful, the side-effects were too severe for widespread use. By the mid 60s, the research had hit a dead end.

Contraceptive research

At roughly the same time, researchers at ICI (now AstraZeneca) in the UK, were investigating the effects of various oestrogen-like chemicals on the reproductive systems of rats, with the aim of trying to find new contraceptives and cholesterol-lowering drugs.

They developed several promising drug candidates, including one with the catchy name ICI46,474. But for various reasons, including lack of support and competing priorities, its development stalled.

War on cancer

Eventually, due to a combination of luck, good judgement, a bit of trans-Atlantic to-ing and fro-ing, and the declaration of a ‘war on cancer’ by President Nixon, there was a renewed interest in developing an oestrogen-blocker to treat breast cancer.

ICI46,474 was developed into tamoxifen, and doctors started giving it to patients in the early 70s. And the drug was licensed for the treatment of advanced breast cancer in the UK in 1972.