Recent studies linking the cancer medication tamoxifen, sold in the United States under the brand name Nolvadex, to the potential risk of venous thromboembolism (pulmonary embolism) has caused Britain's Medicines Control Agency (MCA) to order an investigation of the drug. The MCA's announcement comes after the International Breast Cancer Intervention Study revealed that women taking tamoxifen increased their risk of developing a pulmonary embolism by 50 percent. In March 2002, physicians involved in the international study recommended that women should no longer be prescribed tamoxifen to treat breast cancer. Britain's Committee on Safety of Medicines subsequently issued a statement advising doctors to obtain a family history of pulmonary embolism before prescribing the medication to a patient.

In June 2002, the U.S. Food and Drug Administration and AstraZeneca Pharmaceuticals LP added a boxed warning and strengthened the Warnings section of the label for Nolvadex. Serious, life-threatening or fatal events associated with Nolvadex in the risk reduction setting include endometrial cancer, uterine sarcoma, stroke, and pulmonary embolism. The warning recommends that healthcare providers discuss the potential benefits as well as the potential risks of Nolvadex with women considering taking the medication to reduce their risk of developing breast cancer.