What FDA warning letters to peptide sellers say

Since 2024, FDA has sent warning letters to a series of websites selling peptides labeled for research, most of them GLP-1-class compounds ^2,3,4,5,7. Read side by side, the letters follow a consistent structure. This article summarizes six published letters and what they say, using FDA's wording where it matters. Warning letters are public regulatory records; recipients are named here because FDA publishes them, and a letter states FDA's position and invites a response ¹.

What a warning letter is

FDA describes a warning letter as its usual notice when it identifies what it believes are significant violations of federal requirements; the letter identifies the concerns, gives the company an opportunity to address them and requests a response within a set time ¹. FDA may later issue a close-out letter once it has evaluated a firm's corrective actions ¹. The earlier peptide letters asked for a written response within 15 working days and said that failing to address the matter could lead to legal action including seizure and injunction ^2,3,4.

The letters reviewed

• Synthetix Inc. DBA Helix Chemical Supply, 7 February 2024: semaglutide and tirzepatide ².
• Swisschems, 10 December 2024: semaglutide and retatrutide ³.
• USApeptide.com, 26 February 2025: semaglutide and tirzepatide ⁴.
• Gram Peptides, 31 March 2026: retatrutide, tirzepatide and bacteriostatic water ⁵.
• Wholesale Peptide, 17 June 2026: Prostamax and gonadorelin ⁶.
• Peak Performance Peptides, 24 August 2026: bacteriostatic water, retatrutide sold as "GLP-3R", semaglutide, SS-31, PT-141 and tesamorelin ⁷.

Finding 1: the label does not decide intended use

Every letter sets the seller's disclaimer against the rest of its marketing. In the 2024 letter, FDA noted labeling for "research use only" and "not for human consumption" and said evidence from the website established that the products were intended to be drugs for human use ². Later letters record the phrases "research chemicals only" ³; "research use only," "not for human consumption" and "lab purposes only" together with a standard disease-claim disclaimer ⁴; "Research Use Only" and "not intended for human consumption, medical use, or veterinary use" ⁵; and "RESEARCH USE ONLY" ^6,7. In each case FDA reached the same conclusion about intended use ^3,4,5,6,7. The regulatory background to the phrase is covered in what research use only means.

Finding 2: what counts as evidence of intended use

FDA treats a product as a drug when it is intended to affect disease or the structure or function of the body, and it reads intent from the seller's own materials ^3,4. The evidence cited in these letters falls into a few groups.

• Descriptions that echo approved drugs. One site said its semaglutide was also marketed under the brand names Ozempic, Rybelsus and Wegovy ⁴. Another described retatrutide as "a promising anti-obesity agent" acting on three receptors ³.
• Clinical citations on product pages, such as phase 2 retatrutide trials listed under a "Peer-Reviewed Studies" heading ³.
• Dose information. One site listed dose counts per vial by week and supplied a document that it compared with "the standard schedule for the medication, Mounjaro" ².
• Social media posts promoting the products and directing followers to the store ³.
• Injection supplies. FDA said that selling bacteriostatic water alongside peptides that need reconstitution showed an intent for combined use by injection, which made the water itself a drug ^5,7. A 2025 letter also noted that the site sold insulin syringes ⁴.
• Health claims for non-GLP-1 products, such as statements that Prostamax could improve bladder control and reduce prostate inflammation ⁶.

Finding 3: the charges

The core charge in every letter is that the products are unapproved new drugs: they are not generally recognized as safe and effective for the uses in their labeling, no approved application is in effect, and introducing them into interstate commerce violates sections 301(d) and 505(a) of the Federal Food, Drug, and Cosmetic Act ^2,3,4,5,6,7. Two letters add misbranding. They say the labeling lacks adequate directions for use under section 502(f)(1), because the products are prescription drugs for conditions not suited to self-diagnosis, so directions a layperson could follow safely cannot be written ^2,4. Both of these letters also say that the prescription-drug status of the products misbrands them under section 503(b)(1), and the 2025 letter adds that offering them without a prescription is a prohibited act under section 301(k) ^2,4. The text of the other letters reviewed cites the unapproved-new-drug provisions, although FDA's listing describes several of them as concerning unapproved new drugs and misbranding ^3,6,7.

Finding 4: an approved name does not make a product approved

Some products carried the names of approved drugs. FDA noted that approved semaglutide and tirzepatide products exist but that no approved application covered the seller's versions, and that the approved versions are available only on prescription ⁴. The August 2026 letter listed semaglutide, tesamorelin and PT-141 alongside investigational compounds as unapproved new drugs as sold on that website ⁷.

Finding 5: injection raises the concern

FDA singles out injectable products. A 2025 letter said injectable drugs bypass many of the body's natural defenses against toxic ingredients and organisms, which can lead to serious conditions such as sepsis ⁴. The 2026 letters describe injectable products as especially concerning from a public health perspective for the same reason ^5,6,7.

Code names in the letters

Two 2026 letters show sellers using code names instead of drug names. FDA identified a product sold as "GLP-3R" as retatrutide ⁷. Another letter said the firm referred to retatrutide as "GLP-1-R peptide" and to tirzepatide as "GLP-2 peptide" ⁵. In both letters FDA treated the products as the named drugs and reached the same unapproved-new-drug conclusion ^5,7.

FDA's wider position on research-labeled GLP-1 products

FDA's GLP-1 page says the agency has warned companies that illegally sold semaglutide, tirzepatide, retatrutide, survodutide or mazdutide falsely labeled "for research purposes" or "not for human consumption," and that these products were sold directly to consumers with dosing instructions ⁸. The same page says retatrutide cannot be used in compounding under federal law and has not been found safe and effective for any condition ⁸.

Key points

• A warning letter is FDA's notice of what it considers significant violations, with a request for a response ¹.
• In every letter reviewed, research-only wording did not change FDA's view of intended use ^2,3,4,5,6,7.
• Product descriptions, clinical citations, dose information, social media and injection supplies were all cited as evidence ^2,3,4,5,7.
• The common charge is distribution of unapproved new drugs; some letters add misbranding ^2,4.
• Code names such as "GLP-3R" did not change the analysis ^5,7.