Britain's Medicines Control Agency (MCA) has issued new advice on the use to tamoxifen to doctors, urging them to look at the potential risks of blood clots before prescribing the drug to breast cancer patients.

Tamoxifen is an estrogen receptor antagonist used to treat estrogen receptor positive breast cancer.

The International Breast Cancer Intervention Study, which investigated the drug, has confirmed that women treated with tamoxifen have approximately a 2.3 fold higher risk of venous thromboembolism (blood clots) than those treated with a placebo. Approximately 40 percent of the cases of venous thromboembolism occurred within three months of surgery or following immobility.

The following recommendations were made for the treatment of breast cancer patients using tamoxifen:

Prior to starting treatment with tamoxifen:

• Obtain a careful personal and family history of venous thromboembolism.
• Base the decision to treat on the overall risk to the patient.
• Consider using anticoagulant drug therapy.

In the event of surgery or immobility:

• Do not stop tamoxifen treatment before surgery or long-term immobility unless the risk of venous thromboembolism clearly outweighs the risk of interrupting treatment.
• Consider the possible duration of treatment interruption, the stage and grade of cancer, the clinical response of the patient to tamoxifen therapy and the stage of the treatment regimen at which the interruption occurs.
• Give all patients appropriate thrombosis prevention measures.

In the event of an occurrence of venous thromboembolism:

• Stop tamoxifen immediately and start anti-thrombosis measures.
• Make the decision to re-start tamoxifen with respect to the overall risk to benefit balance for the patient.
• Consider anti-coagulation measures if the patient is to be restarted on tamoxifen.

Other sources: Current Problems in Pharmacovigilance