The U.S. Food and Drug Administration has issued warning letters to Sientra Inc and a Johnson & Johnson unit for failing to comply with the post-approval study requirements for their breast implants. Last month, the FDA issued a statement warning doctors about a type of lymphoma linked to breast implants after receiving reports of the disease. The agency's letters, dated March 18, highlighted issues in the companies' post-approval studies, which included evaluating the long-term performance and safety of the implants, used for breast augmentation and reconstruction procedures.