A warning letter sets out the violations FDA has found, asks for a written response within a stated time (fifteen business days in recent letters to peptide sellers) and warns that further action may follow without notice, including seizure and injunction. It is not a final agency action, but it is published, and it becomes part of the company's record.
FDA's recent letters to research-use-only sellers each quote the seller's disclaimer and then rely on the rest of the website to establish intended use.
FDA describes a warning letter as informal and advisory: it states the agency's position, does not commit it to enforcement action and is not final agency action. The positions in warning letters are the agency's interpretation in particular cases, and they can change.
See also
For in vitro laboratory research only. Not for human or veterinary use. This library describes chemistry, analytical methods and published laboratory research; it gives no guidance on use in people or animals.