A Warning Letter is an official notification from the FDA to a manufacturer that has violated a federal regulation(s). The Warning Letter contains details regarding the specific violations and expects the manufacturer will provide an immediate response outlining their detailed corrective action plan.  The agency may take enforcement action if the corrective action is not implemented.  The letter is usually hand-delivered and the letter is made public.  

The most common FDA violations are:

1. Deficiencies in basic GMP requirements

2. Lack of adequate written procedures and responsibilities for the quality unit

3. Deficiencies in GLP