Lilly's LY numbers: what the drug codes mean

What an LY number is

LY is the prefix on the codes Eli Lilly gives its research compounds. The digits are an internal label. Lilly does not explain how it numbers compounds in any record the Atlas read, so the digits are not a safe guide to age, strength or type.

A code lets scientists, trial registries and regulators refer to a molecule before it has a name. Later, the developer can ask the World Health Organization for an International Nonproprietary Name, the generic name that goes on a label. WHO's expert group examines the proposed name, anyone can comment or object for four months, and if there is no objection the name is published as a recommended name (WHO). A brand name is a third layer, set by the company.

The generic-name steps are from the World Health Organization's description of how an International Nonproprietary Name is requested and selected. The Atlas read the tirzepatide code and name together in the abstract of a 2026 paper (see the next table).

Codes matched to public names

Each row below comes from a record where the code and the name appear together. Journal abstracts and trial-registry entries were read for seven of the matches. The eighth, naperiglipron, is linked to its code by a trade-press report, which is a secondary source.

Targets are as the matching abstract or record states them. A code and name appearing together shows they are the same compound. It says nothing about whether the compound works or is safe. Approval status for the medicines the Atlas covers is on each medicine page.

Why one medicine shows up under two names

Papers and registry records are written before a name exists, and they are not rewritten when one arrives. A 2022 Cell Metabolism paper describes LY3437943 as a triple agonist peptide in its title and abstract without the name retatrutide. The code lingers after the name arrives, too. A 2008 phase 2 record studies LY2189265 in overweight and obese people with type 2 diabetes, and a 2026 abstract still writes dulaglutide (LY2189265).

A code on a registry record also does not show who is running the study. Of the 71 trials in the Atlas pipeline data, pulled October 19, 2026, that name an LY number, 70 list Eli Lilly as sponsor. The other, a phase 2 retatrutide study in China (NCT07467447), lists Hudson Biotech.

A search on one name can miss records that use the other. A registry search that joins the code and the name with OR picks up both.

A code can also name a study or a combined product

Not every LY number is a single molecule. The master protocol that several Lilly weight-loss candidates enter, NCT06143956, has LY900038 in its title (registry record).

Two phase 3 trials, NCT07881432 and NCT07881458, name LY900043 in their titles while listing eloralintide and tirzepatide as the medicines. The first is called AMPLIFY-1, and its summary describes a combined eloralintide and tirzepatide product that it calls EloraTZP (registry record). The second is called AMPLIFY-2 and lists "eloralintide and tirzepatide" as an intervention (registry record). Both are listed as not yet recruiting. The registry does not say what LY900043 stands for, so the Atlas does not either. The eloralintide page has the trial details.

Naperiglipron: two of three phase 2 trials ended

Naperiglipron shows how fast a code can change course. In September 2025 a trade-press report said Lilly had ended two of its three phase 2 trials, one in people with type 2 diabetes and one in adults with a healthy body mass index, and cited business reasons (Fierce Biotech). The registry lists the type 2 diabetes trial as terminated (NCT07030868). It lists the weight-management trial as completed, with 288 people and no posted results (NCT06683508).

The Atlas did not find a public decision on what happens next. Until Lilly or a journal says more, a completed trial with no results is all the record shows.

Lilly codes in weight-related trials with no public name

The registry lists more Lilly codes than there are names. The table covers the codes the Atlas found in weight-related trial records that it could not tie to a public name. Some of these trials have already ended early. A code in a trial does not mean the compound works, is safe or will become a medicine.

Statuses were checked in the registry on October 26, 2026. The record shows what was registered, not what was found. NCT03764774, the 2018 study, is the only one of these records with posted results, and the Atlas has not read them.

What a code does not tell you

A code names a molecule under development. It does not say whether the molecule is a peptide or a pill, whether it works, or whether a regulator has approved it. A compound that is known only by its code is investigational and is not an approved product. The pipeline page tracks the medicines in trials, the medication library gives each one's status, and the brand and generic names page covers the names on labels. Trademark applications are a separate thing again: Lilly has applied to register names such as Retzoundi and Retkouro without saying which medicine they are for, as the Retzoundi and Retkouro page explains.

Quick answers

What does LY stand for?

The Atlas found no record that spells it out. It is the prefix on Eli Lilly's compound codes. Of the 71 trials in the Atlas pipeline data, pulled October 19, 2026, that name an LY number, 70 list Eli Lilly as sponsor.

Does a higher LY number mean a newer drug?

Often, but not always. The oldest code in the matched table, LY2189265, is on a 2008 record, and the newest codes in the unnamed table, LY4174794 and LY4329090, have trials that start in 2026. Eloralintide breaks the pattern. Its code, LY3841136, is a higher number than naperiglipron's LY3549492, yet its first registry trial (NCT05295940) started in March 2022, before the first naperiglipron trial (NCT05327595) in May 2022.

Which Lilly codes match approved medicines?

In the United States, tirzepatide (LY3298176) and orforglipron (LY3502970) are approved for weight management. Retatrutide, eloralintide, nisotirostide, naperiglipron and LY4086940 are investigational, and mazdutide is not FDA approved, though the company says China approved it in 2025. The status of each is on its medication page where the Atlas has one. The Atlas has no page on dulaglutide.

Can I look up a code in a trial registry?

Yes. ClinicalTrials.gov has an intervention field, and a code can be entered there the same way as a generic name. Joining the code and the name with OR catches records that use either.

Is a medicine that is known only by its code available to patients?

Not as an approved product. A compound known only by a code is investigational. The pipeline page lists the trials, and the weight-loss pills guide covers the pills that are approved.

Are Retzoundi and Retkouro Lilly codes?

No. They are trademark applications that Eli Lilly and Company filed with the U.S. Patent and Trademark Office on April 23, 2024. An LY number is a development code for a compound. A trademark is a possible brand name. The USPTO records do not say which medicine either name is for. See the Retzoundi and Retkouro page.

Where can I read about LY4086940?

The LY4086940 page covers the two registered trials and what is not yet public. No dose or results are published and the compound has no brand or generic name, though the Lilly slides list it as GGG Tri-Agonist III. For a reading of the trial records that labels each statement by evidence level, see EloralintideResearch.com. For the public record in date order, see the LY4086940 timeline on PeptideBlog.net.

What is still unknown

• Lilly does not say how it assigns LY numbers in any record the Atlas read.
• The Atlas could not tie ten codes in weight-related trial records to a public name. Some may already have names that the records do not show.
• Only one of the unnamed codes has a registry record with posted results, and the Atlas has not read them. The registry says little about how these compounds performed.
• The registry does not say what LY900043 stands for, or whether it names the combined product, the program or the studies.