The US Food and Drug Administration issued a new warning on the drug tamoxifen, advising doctors that it may cause an aggressive cancer of the uterus.
The labelling of tamoxifen will be changed to add a “black box” warning about the newly identified risk of uterine sarcoma, the drug agency said. Black boxes are used to draw attention to problems that are serious and potentially life threatening.
Letters advising doctors of the new warning were sent out last month, but the information was not posted on the agency's website until Thursday.
Tamoxifen was already known to increase the risk of another, less dangerous type of uterine cancer, endometrial adenocarcinoma, which is usually detected at an early, curable stage. But the risk of the more dangerous type of cancer had not been recognised previously.
The new warning was directed only at women who have not had breast cancer but are at high risk. The warning does not tell those women to avoid tamoxifen, but it does urge them to talk to their doctors about its benefits and risks. The warning does not apply to women who have already had breast cancer and who take tamoxifen to prevent a recurrence. For those women, the FDA said, the benefits far outweigh its risks.
The warning is also aimed at women who have had a very early form of breast cancer that is still confined to the milk ducts (ductal carcinoma in situ, or DCIS). In those two groups, unlike those who have had invasive breast cancer, it has not been proved that tamoxifen prolongs life, even though it does lower the risk of breast cancer.
Uterine sarcoma is rare, estimated to occur in 0.17 women per 1000 a year who take tamoxifen. In women not taking the drug, there are far fewer cases—only 0.01 to 0.02 cases per 1000 women. Since 1978, when tamoxifen was first marketed in the United States, 159 cases of uterine sarcoma worldwide have been reported in women taking the drug.