Long term treatment with tamoxifen has produced few side effects, whic
h are generally mild. Of the serious ones, all of them except eye toxi
city seem to be related to the molecule's intrinsic mildly estrogen-li
ke action, such as, for example, endometrial carcinoma. This property
is also responsible for some favorable clinical effects including a lo
wer risk of osteoporosis and cardiovascular disease. Whether tamoxifen
causes neoplastic growth in patients who develop resistance to this d
rug is still controversial. Further prospective clinical studies are t
herefore needed to investigate such problems and also to evaluate less
frequent side effects. Moreover, decisions on the overall duration of
hormone therapy should be based on possible side effects as well as o
n therapeutic response.