Choice of drug treatment for breast cancer patients, is made by the oncologist. They have a specialized field and statistics they follow based on what they have seen that works for patients.
Response to any drug can be very variable.

Here is a Wiki link to tamoxifen with general information:
http://en.wikipedia.org/wiki/Tamoxifen

This link shows % of side effects reported by doctors and patients to the FDA.

http://www.drugcite.com/?q=tamoxifen
Click on any category and it will expand.

Tamoxifen has been replaced by Femara and Arimidex as far as popularity goes. Why these drugs are selected in various patients depends on the experience of the physician.
Here is the side effect profile for Arimidex:
http://www.drugcite.com/?q=arimidex&a=&s=
for comparison.

Whenever you affect hormone receptors, you are going to get tradeoffs. I don't think that can be avoided.

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