Severe. These medicines may interact and cause very harmful effects and are usually not taken together. Contact your healthcare professional (e.g. doctor or pharmacist) for more information.
How the interaction occurs:
Tamoxifen may speed up how quickly your liver processes anastrozole and letrozole.
What might happen:
The beneficial effects of anastrozole and letrozole may decrease.
What you should do about this interaction:
Let your healthcare professionals (e.g. doctor or pharmacist) know that you are taking these medicines together.Your healthcare professionals may already be aware of this interaction and may be monitoring you for it. Do not start, stop, or change the dosage of any medicine before checking with them first.
1.Antoniou T, Tseng AL. Interactions between antiretrovirals and antineoplastic drug therapy. Clin Pharmacokinet 2005;44(2):111-45.
2.Arimidex (anastrozole) US prescribing information. AstraZeneca Pharmaceuticals LP May, 2013.
3.Dowsett M, Pfister C, Johnston SR, Miles DW, Houston SJ, Verbeek JA, Gundacker H, Sioufi A, Smith IE. Impact of tamoxifen on the pharmacokinetics and endocrine effects of the aromatase inhibitor letrozole in postmenopausal women with breast cancer. Clin Cancer Res 1999 Sep;5(9):2338-43.
4.Nolvadex (tamoxifen citrate) US prescribing information. AstraZeneca Pharmaceuticals LP March, 2006.
5.Weiss L. Personnal communication. Novartis Pharmaceuticals Corporation January 3, 2002.
6.Femara (letrozole) US prescribing information. Novartis Pharmaceuticals Corporation December, 2011.
7.Dowsett M, Cuzick J, Howell A, Jackson I. Pharmacokinetics of anastrozole and tamoxifen alone, and in combination, during adjuvant endocrine therapy for early breast cancer in postmenopausal women: a sub-protocol of the 'Arimidex and tamoxifen alone or in combination' (ATAC) trial. Br J Cancer 2001 Aug 3;85(3):317-24.
8.Dowsett M, Tobias JS, Howell A, Blackman GM, Welch H, King N, Ponzone R, von Euler M, Baum M. The effect of anastrozole on the pharmacokinetics of tamoxifen in post-menopausal women with early breast cancer. Br J Cancer 1999 Jan;79(2):311-5.
9.Bonanni B, Serrano D, Gandini S, Guerrieri-Gonzaga A, Johansson H, Macis D, Cazzaniga M, Luini A, Cassano E, Oldani S, Lien EA, Pelosi G, Decensi A. Randomized biomarker trial of anastrozole or low-dose tamoxifen or their combination in subjects with breast intraepithelial neoplasia. Clin Cancer Res 2009 Nov 15;15(22):7053-60.
10.Ingle JN, Suman VJ, Johnson PA, Krook JE, Mailliard JA, Wheeler RH, Loprinzi CL, Perez EA, Jordan VC, Dowsett M. Evaluation of tamoxifen plus letrozole with assessment of pharmacokinetic interaction in postmenopausal women with metastatic breast cancer. Clin Cancer Res 1999 Jul;5(7):1642-9.