29 C.F.R. § 825.305 : Labor — REGULATIONS RELATING TO LABOR — WAGE AND HOUR DIVISION, DEPARTMENT OF LABOR — OTHER LAWS — THE FAMILY AND MEDICAL LEAVE ACT OF 1993 — EMPLOYEE AND EMPLOYER RIGHTS AND OBLIGATIONS UNDER THE ACT — Certification, general rule

(a) General. An employer may require that an employee's leave to care for the employee's covered family member with a serious health condition, or due to the employee's own serious health condition that makes the employee unable to perform one or more of the essential functions of the employee's position, be supported by a certification issued by the health care provider of the employee or the employee's family member. An employer may also require that an employee's leave because of a qualifying exigency or to care for a covered servicemember with a serious injury or illness be supported by a certification, as described in §§825.309 and 825.310, respectively. An employer must give notice of a requirement for certification each time a certification is required; such notice must be written notice whenever required by §825.300(c). An employer's oral request to an employee to furnish any subsequent certification is sufficient.
(b) Timing. In most cases, the employer should request that an employee furnish certification at the time the employee gives notice of the need for leave or within five business days thereafter, or, in the case of unforeseen leave, within five business days after the leave commences. The employer may request certification at some later date if the employer later has reason to question the appropriateness of the leave or its duration. The employee must provide the requested certification to the employer within 15 calendar days after the employer's request, unless it is not practicable under the particular circumstances to do so despite the employee's diligent, good faith efforts or the employer provides more than 15 calendar days to return the requested certification.
(c) Complete and sufficient certification. The employee must provide a complete and sufficient certification to the employer if required by the employer in accordance with §§825.306, 825.309, and 825.310. The employer shall advise an employee whenever the employer finds a certification incomplete or insufficient, and shall state in writing what additional information is necessary to make the certification complete and sufficient. A certification is considered incomplete if the employer receives a certification, but one or more of the applicable entries have not been completed. A certification is considered insufficient if the employer receives a complete certification, but the information provided is vague, ambiguous, or non-responsive. The employer must provide the employee with seven calendar days (unless not practicable under the particular circumstances despite the employee's diligent good faith efforts) to cure any such deficiency. If the deficiencies specified by the employer are not cured in the resubmitted certification, the employer may deny the taking of FMLA leave, in accordance with §825.313. A certification that is not returned to the employer is not considered incomplete or insufficient, but constitutes a failure to provide certification.
(d) Consequences. At the time the employer requests certification, the employer must also advise an employee of the anticipated consequences of an employee's failure to provide adequate certification. If the employee fails to provide the employer with a complete and sufficient certification, despite the opportunity to cure the certification as provided in paragraph (c) of this section, or fails to provide any certification, the employer may deny the taking of FMLA leave, in accordance with §825.313. It is the employee's responsibility either to furnish a complete and sufficient certification or to furnish the health care provider providing the certification with any necessary authorization from the employee or the employee's family member in order for the health care provider to release a complete and sufficient certification to the employer to support the employee's FMLA request. This provision will apply in any case where an employer requests a certification permitted by these regulations, whether it is the initial certification, a recertification, a second or third opinion, or a fitness for duty certificate, including any clarifications necessary to determine if such certifications are authentic and sufficient. See§§825.306, 825.307, 825.308, and 825.312.
(e) Annual medical certification. Where the employee's need for leave due to the employee's own serious health condition, or the serious health condition of the employee's covered family member, lasts beyond a single leave year (as defined in §825.200 ), the employer may require the employee to provide a new medical certification in each subsequent leave year. Such new medical certifications are subject to the provisions for authentication and clarification set forth in §825.307, including second and third opinions.

29 C.F.R. § 825.305