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While the prototype oestrogen receptor blocker that increased survival in breast cancer, by reducing the risk of developing invasive estrogen receptor-positive breast cancer by 49%[1] it is increasingly being replaced by aromatase inhibitors due to better overall effectiveness. This is partially due to their better side effect profile and the several potential metabolic interactions that on theoretical grounds might be expected to reduce tamoxifen's effectiveness with commonly co-administered drugs, but have been difficult to demonstrate.

Clinical Use

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Place in therapy defined by 2009 NICE guidelines "Postmenopausal women with ER-positive early invasive breast cancer who are not considered to be at low risk should be offered an aromatase inhibitor, either anastrozole or letrozole, as their initial adjuvant therapy. Offer tamoxifen if an aromatase inhibitor is not tolerated or contraindicated."



  • Adjuvant treatment early breast cancer 20mg od
  • In locally advanced or metastatic breast cancer up to 40mg a day.

Clinical Issues

  • Will be protective against osteoporotic fractures. The epidemiological and trial evidence overwhelming favours its place in long term therapy in estrogen receptor-positive breast cancer despite its side effect profile[2].


Cautions and Interactions

  • Monitor for endometrial pathology
  • Baseline opthalmological assessment and reconsider therapy if cataracts and retinopathy develop
  • Interaction via it is believed CYP3A4 and CYP2D6 with now both interactions having evidence of clinical significance:

Side effects

Special advice


Competes for the binding sites with estradiol. Reduces total cholesterol and low density lipoproteins. Increases steroid- and thyroxine-binding proteins. Reduces antithrombin III. N-desmethyltamoxifen is the principal metabolite.