The FDA today released warning letters it sent to Johnson & Johnson (NYSE:JNJ) subsidiary Mentor Worldwide and Sientra (NSDQ:SIEN) over failures to conduct appropriate post-approval studies to analyze the long-term safety and risks associated with their silicone gel-filled breast implants.

The federal watchdog said that every manufacturer of approved silicone gel-filled breast implants is required to conduct post-approval studies of the devices to maintain appropriate data on the long-term safety and potential risks associated with the devices that premarket trials cannot foresee.

In its letter to J&J’s Mentor, the FDA noted a failure to enroll the appropriate amount of patients in the post-market study of its MemoryShape breast implant which won approval in 2013. The federal watchdog also said that Mentor had poor follow-up rates with patients and noted significant data inconsistencies in the report, including poor patient accounting and missing race and ethnicity data.

In its letter to Sientra, the federal watchdog knocked the company on its 61% follow-up rate, which falls below the target for the post-market study.

The agency said that both companies’ failure to address the raised concerns and comply with the post-approval study requirements violates its pre-market approval order. The FDA gave both companies 15 days to comply with the warnings.

“Post-approval requirements are critical to ensuring the safety and effectiveness of the medical products we regulate and we’ll continue to hold manufacturers accountable when they fail to fulfill these obligations. We’re issuing these warning letters based on the manufacturers’ low recruitment, poor data, and low follow-up rates in their required post-approval studies. We expect these manufacturers to meet the pre-specified study requirements in order to ensure the collection of long-term data that can be used to inform long-term patient safety. Post-approval studies, along with other surveillance tools such as adverse event reports, registries, and scientific literature, allow the FDA to help ensure the safety of medical devices and protect patients,” FDA Commissioner Dr. Scott Gottlieb said in a prepared statement.

Last month, the FDA said that it has received reports of 457 unique cancer cases related to breast implants since 2010, including nine patient deaths.