Is retatrutide a drug or a biologic? FDA says drug, and Lilly is still fighting it in court

By · Published · 8 min read · 10 sources, 10 checked at the source

AI-generated image: courtroom table with folders labeled Drug (NDA) and Biologic (BLA), a balance scale, an amino-acid chain diagram marked '39 or 41?', a vial labeled Retatrutide, and a note reading '7th Circuit — awaiting ruling'.

Lilly has said for months that it plans to send FDA a Biologics License Application for retatrutide early next year. FDA has a different view of what retatrutide is. On October 2, 2026, the agency told Lilly again that retatrutide is a drug, not a biologic, so any application would go through the ordinary new-drug route. The disagreement comes down to how you count the building blocks in one molecule, and it's now in front of a federal appeals court.

Facts are as of November 3, 2026. Sources, with retrieval dates, are at the bottom.

Short version

  • FDA's rules call something a "protein," and so a biologic, only if it has a chain of more than 40 alpha amino acids. FDA counts 39 in retatrutide's backbone. [V]
  • Lilly argues the small side chain attached to that backbone should count too, which would push the total past 40. FDA disagrees, because the side-chain pieces aren't alpha amino acids. [V]
  • A federal judge in Indiana sided with FDA on the "protein" question in September 2025 but sent a second question back to the agency. FDA answered it on October 2, 2026, with the same result: not a biologic. [V]
  • Lilly's appeal in the Seventh Circuit was argued on September 24, 2026. We found no ruling as of November 3. Either way, retatrutide is still unapproved everywhere. [V]

Why the label matters

Both paths end at FDA. Drugs get their approvals through the Federal Food, Drug, and Cosmetic Act, while biologics get licenses through the Public Health Service Act. The same FDA center, the Center for Drug Evaluation and Research, would review retatrutide either way, according to FDA's October letter. [V]

The difference that matters to a company is what happens after approval. In its appeals brief, the government acknowledges that biologics usually enjoy a much longer stretch of exclusivity before copies can compete, and it cites the two statutes side by side. [V] Lilly, in its own October 8 filing, says it needs to know which application to file, and that it plans to file by the end of the first quarter of 2027. [V]

How you count to 40

Since March 23, 2020, FDA's regulation has said a protein is a polymer of alpha amino acids, in a specific, set order, longer than 40 units. [V] That rule is the reason insulin and growth hormone moved to biologics licenses that year while most shorter peptides stayed drugs, a shift our explainer on the 2020 polypeptide change covers in detail.

“any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size”

21 CFR 600.3(h)(6), as quoted in FDA's October 2, 2026 letter

Retatrutide sits right at the edge. Here is how each side describes it in the court record:

  • Lilly's count: 41. Its request to FDA described a 39-amino-acid backbone plus a short attached chain of two more units, one gamma-glutamate and one ADO (8-amino-3,6-dioxaoctanoic acid), joined at a lysine. Lilly asked FDA to count every amino acid in the molecule. [V]
  • FDA's count: 39 that qualify. FDA agrees ADO is an amino acid in the chemistry sense. But in an alpha amino acid, one carbon sits between the amino group and the acid group. In ADO, five carbons and two oxygens separate them, so FDA says it doesn't count. The side chain also carries a fatty acid, and FDA treats the whole side chain as outside the count. [V]

So the question isn't really whether retatrutide works, or how big it is in some general sense. It's whether a regulatory definition written for proteins should count a linker that isn't the classic protein building block.

  • 39alpha amino acids in retatrutide's backbone, by FDA's count [V]
  • 41total units Lilly counted, side chain included [V]
  • >40alpha amino acids needed to meet FDA's protein definition [V]

A sibling molecule already took the drug route

FDA's letter points to a Lilly product people know well. Tirzepatide, sold as Mounjaro and Zepbound, also has a backbone of 39 alpha amino acids, carrying ADO and gamma-glutamate in its side chain, and Lilly identified it as a related product in its request. [V] Tirzepatide was approved through new drug applications, NDAs 215866 and 217806, and FDA says it treated ADO the same way there. [V]

FDA also names a case where the count went the other way. Tesamorelin, sold as Egrifta, has an unusual group at one end, but FDA wrote in 2020 that its other 43 amino acids are all alpha amino acids, which is enough to make it a protein. Its drug approval was deemed a biologics license on March 23, 2020. [V] Our tesamorelin page covers that drug's trials.

How the case got here

  1. January 2024

    Lilly asks FDA to classify retatrutide

    Lilly files a request for designation asking that retatrutide be treated as a biologic. FDA dates the request January 19 in its 2026 letter.

  2. March 18, 2024

    FDA says drug

    FDA's first answer: retatrutide is neither a protein nor an "analogous product" to one.

  3. September 3, 2024

    Lilly sues

    Lilly challenges the decision in federal court in the Southern District of Indiana.

  4. September 30, 2025

    Split ruling

    The judge upholds FDA on the protein question but vacates the "analogous product" finding and sends it back to FDA.

  5. September 24, 2026

    Appeal argued

    The Seventh Circuit hears Lilly's appeal and asks both sides for extra briefs on whether it has jurisdiction.

  6. October 2, 2026

    FDA answers again

    On remand, FDA concludes retatrutide still isn't analogous to a protein and isn't a biologic.

  7. October 8, 2026

    Briefs filed

    The government argues Lilly can't appeal yet. Lilly argues it can, and that only the appeals court can give it the relief it wants.

The judge's ruling explains the second question. Even a molecule that misses the protein definition might count as "analogous" to a protein, and the court found FDA hadn't spelled out a consistent test for that. [V] On remand, FDA named two features it says any analogous product must share with a protein: a chain, or linked chains, of more than 40 alpha amino acids, and a specific, defined sequence. Retatrutide fails the first, FDA concluded, so it isn't analogous either. [V]

Lilly is also pressing a timing argument. A federal statute gives FDA 60 days to answer a classification request, and if FDA doesn't respond in time, the sponsor's recommendation takes effect. Lilly argues that since FDA's 2024 answer was later thrown out, its own recommendation should stand. FDA's reply is that it answered on March 18, 2024, inside the window, and that the statute doesn't cover answers a court later vacates. [V]

The government's October 8 letter argues something narrower first. It says a private company generally can't appeal an order sending a case back to an agency until the agency is done and the district court has reviewed the result. [V] Lilly's brief counters that Seventh Circuit precedent allows these appeals, and that a new lawsuit couldn't revive the arguments the district court already rejected. [V]

What doesn't change

None of this makes retatrutide available. Lilly calls it an investigational molecule not approved by FDA or any regulator anywhere, and says anyone selling it to consumers is breaking the law. [V] FDA says retatrutide can't be used in compounding under federal law. [V] HHS's six-peptide compounding policy doesn't cover it. Our retatrutide compounding analysis explains why.

The classification fight is about paperwork for a future application, and the trial record stands apart from it. For the trials, see our retatrutide evidence page.

Peptide Policy Watch

Is retatrutide a drug or a biologic? FDA says drug, and Lilly is still fighting it in court

Published November 3, 2026 · https://peptidepolicywatch.com/2026/11/03/retatrutide-drug-or-biologic-fda-39-amino-acids-lilly-appeal/

Printable checklist

Questions for your clinician

Questions this page raises, worth bringing to a clinician or pharmacist familiar with your health history.

  1. Is the medicine being discussed FDA-approved, and for what use?
  2. If someone offers retatrutide now, how is that possible when it's still in trials?
  3. Would joining a clinical trial be an option, and where are trials listed?

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Gaps in what we know

  • We read the September 2025 district court opinion, the government's May 2026 appeals brief, the October 2026 FDA letter and both October 8 filings. FDA's letter is redacted in places. We haven't read Lilly's opening brief or the hearing recording. [U]
  • The court documents we could open run through October 8, 2026. A docket search didn't show a ruling as of November 3, but we couldn't load the full docket page. [U]
  • Whether a biologic label would change retatrutide's market position, and by how much, is a business question. We report only what the filings say. [U]

If the appeals court rules for Lilly, the 40-amino-acid line would get its first big test since 2020. If it rules for FDA, or decides it can't hear the case yet, Lilly's planned filing would, as things stand, go in as a new drug application. Either way, we'll update this page and the peptide policy tracker when a ruling comes.

Bar chart titled Where the 40-amino-acid line falls. A red line marks 40 amino acids, FDA's 2020 dividing line between peptides, which are regulated as drugs, and proteins, which are biologics. BPC-157 has 15 amino acids and sermorelin 29, both on the peptide side under drug rules. Tesamorelin has 44, insulin 51 counting both chains, and somatropin 191; all three moved to biologics in 2020. Somatropin's bar is shortened to fit.
Diagram: chain lengths of five familiar molecules against FDA's 40-amino-acid line.

Sources (retrieved November 3, 2026 unless noted)

  1. FDA, Center for Drug Evaluation and Research, letter to Eli Lilly and Company, "Issues Remanded to FDA," October 2, 2026 (redacted; filed October 8, 2026 as Doc. 46-2, 7th Cir. No. 26-1301) [V] — https://storage.courtlistener.com/recap/gov.uscourts.ca7.55916/gov.uscourts.ca7.55916.46.2.pdf
  2. Eli Lilly and Co. v. Kennedy, No. 1:24-cv-01503, S.D. Ind., order on cross-motions for summary judgment, September 30, 2025 [V] — https://www.govinfo.gov/content/pkg/USCOURTS-insd-1_24-cv-01503/pdf/USCOURTS-insd-1_24-cv-01503-0.pdf
  3. U.S. Department of Justice, brief for the appellees, 7th Cir. No. 26-1301, filed May 15, 2026 (Doc. 23) [V] — https://storage.courtlistener.com/recap/gov.uscourts.ca7.55916/gov.uscourts.ca7.55916.23.0.pdf
  4. U.S. Court of Appeals for the Seventh Circuit, order directing supplemental memoranda on appellate jurisdiction, September 24, 2026 (Doc. 45) [V] — https://storage.courtlistener.com/recap/gov.uscourts.ca7.55916/gov.uscourts.ca7.55916.45.0.pdf
  5. U.S. Department of Justice, supplemental letter brief for the government, October 8, 2026 (Doc. 46-1) [V] — https://storage.courtlistener.com/recap/gov.uscourts.ca7.55916/gov.uscourts.ca7.55916.46.1.pdf
  6. Eli Lilly and Company, supplemental brief of appellant, October 8, 2026 (Doc. 47) [V] — https://storage.courtlistener.com/recap/gov.uscourts.ca7.55916/gov.uscourts.ca7.55916.47.0.pdf
  7. Eli Lilly, "Lilly's retatrutide delivered greater A1C reduction and more than double the weight loss of Ozempic's highest approved dose in head-to-head trial," October 28, 2026 [V] — https://investor.lilly.com/news-releases/news-release-details/lillys-retatrutide-delivered-greater-a1c-reduction-and-more
  8. Eli Lilly, TRIUMPH-2 and TRIUMPH-3 release, July 23, 2026 (retrieved October 29, 2026) [V] — https://investor.lilly.com/news-releases/news-release-details/lillys-triple-agonist-retatrutide-successful-two-additional
  9. FDA, "FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss" (retrieved October 29, 2026) [V] — https://www.fda.gov/drugs/drug-alerts-and-statements/fdas-concerns-unapproved-glp-1-drugs-used-weight-loss
  10. FDA, "Definition of the Term 'Biological Product,'" final rule, 85 FR 10057, February 21, 2020 (FR Doc. 2020-03505) [V] — https://www.federalregister.gov/d/2020-03505
Cite or link to this page

Peptide Policy Watch staff. “Is retatrutide a drug or a biologic? FDA says drug, and Lilly is still fighting it in court.” Peptide Policy Watch. Published November 3, 2026. https://peptidepolicywatch.com/2026/11/03/retatrutide-drug-or-biologic-fda-39-amino-acids-lilly-appeal/

Link: https://peptidepolicywatch.com/2026/11/03/retatrutide-drug-or-biologic-fda-39-amino-acids-lilly-appeal/

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