Aromatase inhibitors like AstraZeneca (nyse: AZN)'s tamoxifen were revolutionary in the treatment of breast cancer.
These women started taking letrozole anywhere from one to seven years after they stopped taking tamoxifen.
For another, tamoxifen mimics the effects of estrogen in some ways and Arimidex doesn't.
"It has been estimated that 350, 000 women are alive today because of tamoxifen, " Jordan says.
But tamoxifen can cause cancer of the lining of the uterus in post-menopausal women.
Tamoxifen has been a vital part of doctors' arsenal against breast cancer for 20 years.
Tamoxifen blocks the female sex hormone oestrogen that fuels the growth of some breast tumours.
BBC: Scientists find why tamoxifen fails some breast cancers
Tamoxifen is given to most women diagnosed with breast cancer to prevent the cancer returning.
BBC: Scientists find why tamoxifen fails some breast cancers
Tamoxifen is not suitable for every woman and can cause side-effects, like hot flushes.
For one thing, tamoxifen can be used on women who have not reached menopause, while Arimidex can't.
Tamoxifen keeps cells from using estrogen, so it can keep most kinds of breast cancer from growing.
AstraZeneca leads a pack of drugmakers that have developed follow-up drugs, but tamoxifen remains the gold standard.
Every day for six months, 51 women took 20mg tamoxifen pills, and 60 took a placebo pill.
"Let's not diminish the effect of tamoxifen by this side effect, " said Peters, referring to endometrial cancer.
CNN: Scientists confirm tamoxifen works but say it isn't for everyone
Like tamoxifen, this drug blocks the action of oestrogen, but it tends to have fewer side effects.
Because hormone therapy is less toxic than chemotherapy, your doctor might start with tamoxifen or an aromatase inhibitor.
Also, over decades of experience doctors have gotten to know the ups and downs of tamoxifen extremely well.
Tamoxifen works by substituting for estrogen in breast tissue, hogging all the estrogen receptors on breast cancer cells.
Previous breast cancer trials have reported 15% to 60% reductions in cardiac death in women treated with tamoxifen.
All of the women involved in the study had been taken tamoxifen for between two to three years.
"For the majority of women, it's a low risk, but tamoxifen carries a lot of emotional baggage, " she says.
No matter what future clinical trials show, Arimidex will never fully replace tamoxifen.
Women who carry genes linked to breast cancer can reduce their risk of developing the disease by taking tamoxifen.
Once FGFR1 was stopped, hormone-based treatments like Tamoxifen could get back to work in destroying cancer cells, they found.
BBC: Scientists find why tamoxifen fails some breast cancers
Taking tamoxifen for five years could cut their own risk of the disease.
This means that women with osteoporosis may see their bones get much weaker on Arimidex than they would on tamoxifen.
Other tests showed it could be used after taking tamoxifen to further reduce the risk of the disease coming back.
Research suggests that up to half of women given tamoxifen to stop a breast cancer returning stop taking the drug prematurely.
But it doesn't make the London-based drug giant much money anymore: Tamoxifen went off-patent and is now available as a cheap generic.
Patients who took Femara for five years had 19% fewer breast cancer recurrences than those who took Tamoxifen, the current gold standard.
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