Tirzepatide CAS Number and What It Reveals About the Molecule

Why the CAS number 2023788-19-2 matters in a clinical and regulatory context

A CAS number is an unambiguous chemical fingerprint. For tirzepatide, CAS 2023788-19-2 refers to a synthetic 39-amino-acid peptide engineered to activate two gut-hormone receptors simultaneously: the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor. No other approved medicine shares this identifier. That dual-action profile is precisely what distinguishes tirzepatide from semaglutide, which acts only on GLP-1 receptors.

In research databases, journal papers and regulatory dossiers, the CAS number is how scientists and reviewers refer to the molecule unambiguously across languages and brand names. When NICE evaluated tirzepatide for weight management and published its recommendation in December 2024 (NICE technology appraisal TA1026) the underlying clinical evidence was tied to this specific molecule. The SURMOUNT-1 trial, published in the New England Journal of Medicine, studied tirzepatide (CAS 2023788-19-2) across 2,539 adults with obesity and found average body-weight reductions of around 20–21% at the 15 mg maintenance dose over 72 weeks.

For a patient, the practical significance is narrower but still real: if you see the CAS number cited by a seller as a quality credential, it tells you the molecule being described, not that the product in the pen is genuine, unadulterated, or legally supplied. Those assurances come from a different set of checks entirely, which are covered below.

What the CAS number cannot tell you about a pen's authenticity

A question our prescribers hear most weeks runs something like: "I've seen a pen online with all the right numbers on it, how do I know it's real?" The CAS number is not part of the answer. Mounjaro KwikPens carry a lot number, an expiry date, and a serialised code linked to Eli Lilly's authorised supply chain. Those are the identifiers that matter for product-level verification. The Mounjaro serial number check page explains how that process works in practical terms.

The MHRA has repeatedly warned about fake weight-loss pens circulating through unregulated online channels and social media. In October 2025, enforcement teams raided the first illicit UK manufacturing site producing counterfeit tirzepatide pens, seizing roughly 20 million doses of illegally traded weight-loss medicines with an estimated street value of around £45 million. Counterfeit pens have contained insulin and other substances in place of the active ingredient. A label printed with a correct CAS number costs almost nothing to produce. Verification that a pen is genuine requires proof that it moved through the licensed UK supply chain, dispensed by a GPhC-registered pharmacy against a valid prescription. You can confirm any online pharmacy's registration on the GPhC pharmacy register.

Reporting suspect sellers or adverse reactions from a product you cannot verify is straightforward via the MHRA Yellow Card scheme.

From molecule to licensed medicine: how tirzepatide reached UK patients

The journey from CAS 2023788-19-2 to a pen on a pharmacy shelf is long. Pre-clinical studies established that the molecule's dual-receptor activity produced meaningful effects on appetite signalling, gastric emptying and blood glucose. The SURMOUNT clinical programme then tested it across thousands of adults. NICE reviewed that evidence against the UK's cost-effectiveness threshold and recommended tirzepatide for weight management in adults with a BMI of at least 35 plus at least one weight-related comorbidity, with lower BMI thresholds applying for some ethnic backgrounds per UK guidance. Researchers are also investigating the molecule's potential beyond weight management, and our page on tirzepatide for MCAS covers what the emerging evidence suggests for people with mast cell activation syndrome. The full tirzepatide overview covers the mechanism, dosing schedule and licensed eligibility criteria in more detail.

The licensed product in the UK is Mounjaro, a once-weekly subcutaneous injection available as a pre-filled KwikPen in six strengths from 2.5 mg to 15 mg. Treatment begins at 2.5 mg (a dose chosen to let the body adjust, not as a therapeutic target) and is titrated by the prescriber in roughly four-week steps. The NHS tirzepatide medicine page gives a plain-English overview of the medicine alongside safety information written for patients.

For people exploring whether Mounjaro suits their circumstances, the weight-loss treatment overview sets out how the private prescription route works and what the consultation covers. Cost context for private treatment in the UK is covered on the Mounjaro price comparison page, which explains what legitimate treatment typically includes beyond the pen itself.

Practical safety considerations tied to the molecule's properties

Tirzepatide's mechanism drives most of its safety profile. Because it slows gastric emptying and acts on appetite centres, the most common side effects are gastrointestinal: nausea, vomiting, diarrhoea, constipation and reflux. These tend to peak after starting or after a dose increase, then settle over days to two weeks for most people. Injection-site reactions, fatigue and headache are also reported. The medicine carries a Black Triangle (▼) designation in the UK, meaning the MHRA requires additional post-marketing surveillance data.

Pancreatitis is a less common but serious risk. Seek urgent medical attention for severe stomach pain that spreads to the back, particularly if accompanied by vomiting. Tirzepatide is not recommended during pregnancy, while breastfeeding, or for anyone trying to conceive. It is not licensed for under-18s. Women taking oral contraceptives should discuss contraceptive method with their prescriber at the start of treatment and after each dose increase, as absorption may be affected.

None of these details change based on the CAS number, they are properties of the molecule itself, confirmed through clinical trials and reflected in the Mounjaro guidance our prescribers apply at consultation. If you want to understand how these factors apply to your own health picture, speaking to one of our clinical team is the right next step, and you can find all the ways to reach us on our Mounjaro contact number page. You can read more about our approach on the about us page, or start your free consultation at any point.