Robotic surgery was never approved for mastectomy or any other cancer-related treatment, but that has hardly deterred doctors in the operating suite. The equipment is widely used to operate on patients with various malignancies, from breast cancer to prostate cancer.

Yet there have long been questions about how well doctors are trained on the machines, and whether the devices are better for patients than traditional methods.

Now the Food and Drug Administration has warned that there is no evidence cancer patients receiving robotic procedures live longer than those who have traditional procedures. And some research shows that patients with cervical cancer fare worse.

“We want doctors and patients to be aware of the lack of evidence of safety and effectiveness for these uses so they can make better informed decisions about their cancer treatment and care,” said Dr. Terri Cornelison, assistant director for the health of women at the F.D.A.’s Center for Devices and Radiological Health.