Retatrutide pen: what's inside and what the label means

What it is

A retatrutide pen is a pre-filled injection pen that holds retatrutide already dissolved in liquid, inside a glass cartridge. Retatrutide is an experimental peptide from Eli Lilly that acts on three hormone receptors at once: GLP-1, GIP and glucagon. A receptor is a docking point on a cell, and a molecule that switches it on is called an agonist, so retatrutide is described as a triple agonist (Jastreboff 2023). In the clinical trials it was given by injection under the skin once a week. The pens sold for research in Europe are made by independent manufacturers. Lilly does not sell retatrutide, and no regulator had approved it as a medicine as of November 2026 (Lilly, July 2026).

The full summary of every published trial is in our main guide: Retatrutide: what the studies say. This page answers the questions people ask about the pen itself.

What the label means

Research pens are labelled with their total content, usually 15 mg or 40 mg. That number is the whole amount of peptide in the cartridge. It is not a dose and it does not describe how the contents would be divided.

Three numbers are useful to tell apart:

• Total content (for example 40 mg): the amount of peptide in the whole pen.
• Concentration (for example 14 mg per ml): how much peptide is in each millilitre of liquid. Total content divided by the liquid volume gives the concentration.
• Purity (for example 99.8 %): the share of the sample that is the intended molecule, measured by HPLC, a laboratory method that separates a sample into its components.

A label alone proves none of these. A report from an independent laboratory, tied to a batch number and checkable online, is what turns a label into a fact.

What the studies used

Every number below comes from trials run under medical supervision. They are reported here as study facts, not as instructions.

• Phase 2, obesity. 338 adults received 1 mg, 4 mg, 8 mg or 12 mg once a week for 48 weeks. Some groups started at 2 mg or 4 mg and stepped up every few weeks. At 48 weeks the mean weight change was −24.2 % with 12 mg against −2.1 % with placebo, an injection with no active ingredient (Jastreboff 2023).
• TRIUMPH phase 3 programme. Over 5,800 participants in four trials received 4 mg, 9 mg or 12 mg once a week (Giblin 2026). In TRIUMPH-1 the mean weight change at 80 weeks was −25.0 % with 12 mg against −3.9 % with placebo (Jastreboff 2026).

The trial drug was supplied by Lilly in its own presentation. The research pens on the market are a different product, and the trials say nothing about them directly.

Pen or vial: what changes

Retatrutide for research comes in two formats.

• Vials hold a freeze-dried powder, also called lyophilised. Before use the powder is dissolved in a measured volume of sterile or bacteriostatic water, a process called reconstitution. The concentration then depends on the volume added, so the arithmetic is done by hand each time. Our reconstitution calculator shows how the numbers work.
• Pre-filled pens arrive already in solution, at a concentration fixed by the manufacturer. There is no powder, no water to add and no calculation of concentration. The cartridge is read through a dial on the pen.

The trade-offs, including stability and handling, are explained in Pen or vial?.

What the researchers flagged

The safety findings below come from the trials of the compound, not from tests of any pen.

• The most common adverse events were digestive: nausea, diarrhoea, constipation and vomiting. In phase 2 they were dose-related, mostly mild to moderate, and partly reduced when the starting amount was lower (Jastreboff 2023).
• Heart rate rose with the dose in phase 2, peaked at 24 weeks and then declined (Jastreboff 2023).
• The longest published follow-up is 80 weeks. What happens after stopping, and over many years, is not yet known.

The full list, trial by trial, is on Retatrutide side effects: what the trials reported.