Its stock price dropped a little more than four percent on the F.D.A. news.

Last September, the Securities and Exchange Commission accused its former chief executive, Hani Zeini, of concealing damaging information about the manufacturer of its implants before closing a $60 million stock offering in 2015. The Brazilian manufacturer had had a certificate of compliance required for selling in the European Union suspended. Sientra said its issue with the S.E.C. had been resolved.

The other letter went to Mentor Worldwide, owned by Johnson & Johnson and based in Irvine, Calif. The F.D.A. said the company had not enrolled enough patients in a study of its MemoryShape implant, approved in 2013, and also threatened to rescind approval for the product. Withdrawing approval for a medical device is time-consuming and rarely occurs.

Mindy Tinsley, a spokeswoman for Mentor, said the company was disappointed by the F.D.A.'s decision to issue a warning letter "despite our good faith efforts to address post-approval study requirements." She said Mentor notified the FDA last year that it would fall short of study enrollment targets because of changes in consumer preferences, but did not hear back. J&J’s stock remained unaffected on Tuesday.

According to the F.D.A., the other two manufacturers whose breast implants are approved in the United States are Allergan and Ideal Implant, which did not receive warning letters on Tuesday. But their implants have also drawn some illness-related complaints from women.

Plastic surgeons and implant manufacturers often say breast implants are the most intensely studied of all medical devices. But critics and patient advocates say most studies done to date are flawed.

“When plastic surgeons tell women that ‘this is the most studied medical device in the world,’ women assume that means they are proven safe,” said Diana Zuckerman, president of the National Center for Health Research in Washington, D.C., who has been advising advocates for patients with implant-related illness. But, “we still don’t know what percentage of women become seriously ill from their breast implants. We still don’t know why some women get sick right away, some get sick years later and some never get sick.”

Dr. Zuckerman, trained in psychology and epidemiology, will speak at the F.D.A. meeting next week. She wrote a 40-page analysis of breast implant studies, and found that most had not tracked long-term outcomes, or had lost too many participants. In addition, she said, the studies focused only on diseases with specific diagnoses, while ignoring symptoms like joint pain and chronic fatigue. And they were generally too small to detect rare diseases, and were funded by implant manufacturers or plastic surgery associations that had a stake in the outcomes.