General Rules. Employers may require certification of the need for leave related to a serious health condition of the employee or a family member, and leave for qualifying exigencies and military caregiver leave. In most cases, this certification should be requested at the time the employee gives notice of the need for leave or within five business days thereafter (this is included in the prototype Notice of Eligibility and Rights and Responsibilities, Form WH-381), but the employer may request certification at a 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 within 15 calendar days after the employer’s request, unless it is not practicable despite the employee’s diligent, good faith efforts.
The employee must provide a complete and sufficient certification. The certification is incomplete if one or more of the applicable entries have not been completed. It is insufficient if the information provided is vague, ambiguous or non-responsive. If the certification is incomplete or insufficient, the employer must advise the employee in writing of the information necessary to make it complete or sufficient and give the employee seven calendar days (unless impracticable) to cure the deficiency.
At the time the certification is requested the employer must advise the employee of the consequences of failure to return the certification. If an employee fails to provide complete and sufficient certification despite an opportunity to cure, or fails to provide any certification, the employer may deny FMLA coverage until the required certification is provided. If the employee never provides the certification, the leave is not protected under the FMLA.
 29 C.F.R. § 825.305.