Femara/tamoxifen

AIs don’t cause blood clots or uterine cancer, and have a slightly lower incidence of hot flashes than tamoxifen. But they do weaken bones, and they’re likelier to cause bone & joint pain, thin hair, and dry skin. Tamoxifen doesn’t weaken bones--it strengthens them; but it can cause uterine cancer (and after prophy hysterectomy, possibly retroperitoneal cancer instead), blood clots and cause or accelerate ripening of cataracts. They work in different ways--tamoxifen, to put it simplistically, “clogs” the tumor’s estrogen receptors so it can’t get the estrogen your body still makes (by ovaries if premenopausal, or fat cells & adrenals if postmenopausal). So your body in general isn’t as deprived of estrogen as it is with AIs. AIs prevent the enzyme aromatase from completing the process of converting the androgens produced by the fat cells & adrenals into estrogen. Both drugs starve the tumor cells of estrogen--tamox by “wiring the tumor’s jaws shut” and AIs by "cleaning out & padlocking the fridge.” Both drugs work in premenopausal women, but AIs deprive the body of estrogen--which can be too abrupt a transition into menopause for younger women. OTOH, AIs are more effective in postmenopausal women because the estrogen depletion isn’t as abrupt in women whose levels are already low, but it’s more complete; it also doesn’t accelerate the cardiovascular problems of older women.

One other consideration--AIs have fewer drug interactions, especially with antidepressants. Dopamine agonists with “transparent” SE profiles (i.e., fewer side effects) like bupropion are okay with AIs, as are SSRIs, SNRIs and tricyclics. But bupropion and most SSRIs & SNRIs (except Effexor) inhibit the enzyme pathway by which tamoxifen blocks estrogen receptors. Since not everyone responds to the same antidepressant in the same way and it often takes trying two or three to find one that works (I’m a veteran of this stuff), if you already have or are prone to depression and find yourself developing it during your cancer “journey," you have more pharmacological wiggle room if you take an AI. (Or so my shrink explained).