A retatrutide pen contains the substance already dissolved in a liquid. A vial contains retatrutide as a lyophilised (freeze-dried) powder, which has to be dissolved in the lab before you can work with it. At Clean Peptides, both forms contain the same amount of retatrutide per pack: 15, 20, 30 or 40 mg. The real difference comes down to three things: whether a preparation step is needed, how stable the material is and how you store it.
Retatrutide (research code LY3437943, often shortened to ‘reta’) is a synthetic peptide developed by Eli Lilly that is still being studied in clinical trials and has no marketing authorisation. You can read more about the substance and the state of research on our retatrutide page.
The differences at a glance
| Feature | Pre-filled pen | Vial |
|---|---|---|
| Form | Retatrutide dissolved in liquid | Lyophilised powder in a glass vial |
| Content per pack | 15, 20, 30 or 40 mg retatrutide | 15, 20, 30 or 40 mg retatrutide |
| Reconstitution required | No | Yes, with a solvent sold separately |
| Concentration | Fixed at filling | Your choice, set by the volume of solvent |
| Storage (as per label) | 2–8 °C, dry, protected from light | Unopened: 2–8 °C, dry, protected from light |
| Main risk | Heat, freezing and shaking | Moisture and condensation |
In both cases, the mg figure is the total amount of retatrutide in the pen or vial, not a concentration. Two products labelled ‘15 mg’ therefore contain the same amount of retatrutide, even though their packaging looks very different.
What is inside a pen and what is inside a vial
For a vial, the solution is freeze-dried after filling, a process also known as lyophilisation. The water is first frozen and then removed under vacuum. What remains is a dry, usually white cake or powder at the bottom of the glass vial, which is sealed with a stopper and a cap.
In a pen, retatrutide is dissolved in the liquid with which the pen is filled. The excipients this liquid contains besides water, such as a buffer, vary from manufacturer to manufacturer. If the composition matters for your study design, ask us about it before you order.
Reconstitution: when it is and is not needed
The powder in a vial always has to be dissolved first. Which solvent is suitable depends on the peptide and on the experiment. Common options are bacteriostatic water, which contains a preservative, and sterile water without additives. We sell the solvent separately, as bacteriostatic water 3 ml and 10 ml. The advantage of a vial is that you decide how much solvent to use, and therefore the concentration of your stock solution.
A pen needs no reconstitution. That saves a preparation step, and every preparation step is an opportunity for contamination [1]. On the other hand, you cannot change the concentration or the composition of the liquid.
Stability: powder and liquid behave differently
Peptides are less stable in water than in dry form. In solution they can hydrolyse (the chain breaks), deamidate (a side chain changes), oxidise and clump together (aggregate). Temperature and pH determine how quickly this happens, and freezing, heating and shaking can also trigger aggregation [1]. This is why many peptide medicines are stored and transported in a cold chain [1].
Dry powder is less affected by this, but it is sensitive to something else: moisture. In a classic study of lyophilised insulin, aggregation increased as the powder absorbed more water from its surroundings [2]. Peptide powders often attract moisture, so manufacturers advise letting a cold pack warm to room temperature before opening it [3].
There are no public stability data for retatrutide as research material, so the storage advice on the label and general knowledge about peptides are the starting point.
Storage and transport
According to the label, the same applies to both forms: store refrigerated at 2–8 °C, dry and protected from light. Do not freeze a pen, because freezing can promote aggregation in the solution [1]. Keep a vial closed until you need it. How long the material remains usable after reconstitution, and how to handle light and transport, is explained in How to store retatrutide: temperature, light and shelf life.
We ship from the Netherlands with tracking. Within the Netherlands, a parcel is usually delivered one day after dispatch. Put the pen or vial in the fridge as soon as it arrives. Delivery times for each country are listed on the Shipping page.
Choosing a pen or vial for lab research
Which form suits you best depends on your study design. These questions can help you choose:
- Do you need a specific concentration or a specific solvent? Then a vial is the obvious choice, because you prepare the solution yourself.
- Do you want to skip the preparation step? A pen contains a ready-made solution at a fixed concentration.
- When will you use the material? If the experiment is some time away, dry powder is generally the more stable form to keep in stock [1].
- What do you want to have analysed? For a vial, the analysis covers the powder; for a pen, the filled solution. How to assess such an analysis is explained in How to read a certificate of analysis (COA).
Both forms come in four strengths. The pen: 15 mg, 20 mg, 30 mg and 40 mg. The vial: 15 mg, 20 mg, 30 mg and 40 mg. Prices and stock levels are shown on the retatrutide overview page. Questions about ordering, payment and shipping are answered in the frequently asked questions.
Further reading
- Retatrutide: overview and strengths
- How to store retatrutide: temperature, light and shelf life
- Retatrutide vs semaglutide and tirzepatide: the differences
- What is bacteriostatic water and what is it used for in the lab?
Sources
- Nugrahadi PP, Hinrichs WLJ, Frijlink HW, et al. Designing Formulation Strategies for Enhanced Stability of Therapeutic Peptides in Aqueous Solutions: A Review. Pharmaceutics 2023;15(3):935. doi:10.3390/pharmaceutics15030935
- Costantino HR, Langer R, Klibanov AM. Moisture-induced aggregation of lyophilized insulin. Pharmaceutical Research 1994;11(1):21–29. doi:10.1023/a:1018981208076
- Bachem. Handling and Storage Guidelines for Peptides. bachem.com (accessed 30 September 2026)
For laboratory research only. Clean Peptides products are not intended for use in humans or animals. This article provides general information and is not medical advice.
