How to reconstitute retatrutide

If you have obtained retatrutide from a compounding pharmacy in lyophilized (freeze-dried) form, you will need to reconstitute it with bacteriostatic water before injection. This process is straightforward if you follow the steps carefully, but getting the math wrong means incorrect dosing, which can increase side effects or reduce effectiveness.

This guide walks through reconstitution, dosing calculations, storage, and common mistakes. If you have never mixed a peptide before, start with our general how to reconstitute peptides guide for background on the supplies and technique.

A note on context: Retatrutide is not FDA-approved as of April 2026. If you have received it from a compounding pharmacy, it was prepared under a physician’s order. The reconstitution steps below apply to compounded lyophilized powder forms. Clinical trial retatrutide comes in prefilled pens that do not require mixing. For more on how to access retatrutide, see our access guide.

Key takeaways

• Reconstitute retatrutide with bacteriostatic water, not sterile water (bacteriostatic water allows multi-dose use)
• The amount of water you add determines the concentration and therefore your injection volume per dose
• Common vial sizes are 5 mg, 10 mg, and 15 mg of lyophilized retatrutide
• Store reconstituted retatrutide in the refrigerator (36-46°F / 2-8°C)
• Reconstituted vials are good for up to 28 days when using bacteriostatic water
• Never shake the vial; swirl gently to dissolve

What you need

Before starting, gather everything:

• Retatrutide lyophilized vial (check the label for total peptide content in mg)
• Bacteriostatic water (BAC water) vial
• Insulin syringes (1 mL / 100 unit) for drawing and injecting
• Alcohol swabs
• A clean, flat workspace

Insulin syringes marked in “units” are standard for peptide dosing. A 1 mL syringe equals 100 units. Every 10 units on the syringe equals 0.1 mL.

Step-by-step reconstitution

Step 1: Choose your water volume

The volume of bacteriostatic water you add determines the concentration. More water means a lower concentration and a larger injection volume per dose. Less water means a higher concentration and smaller injections.

Here are common reconstitution ratios for different vial sizes:

5 mg vial:

BAC water added	Concentration	Volume for 2 mg dose	Volume for 4 mg dose
1 mL	5 mg/mL	0.4 mL (40 units)	0.8 mL (80 units)
2 mL	2.5 mg/mL	0.8 mL (80 units)	Not practical (1.6 mL)

10 mg vial:

BAC water added	Concentration	Volume for 2 mg dose	Volume for 4 mg dose	Volume for 8 mg dose
1 mL	10 mg/mL	0.2 mL (20 units)	0.4 mL (40 units)	0.8 mL (80 units)
2 mL	5 mg/mL	0.4 mL (40 units)	0.8 mL (80 units)	Not practical
5 mL	2 mg/mL	1.0 mL (100 units)	Not practical	Not practical

15 mg vial:

BAC water added	Concentration	Volume for 4 mg dose	Volume for 8 mg dose	Volume for 12 mg dose
1.5 mL	10 mg/mL	0.4 mL (40 units)	0.8 mL (80 units)	Not practical (1.2 mL)
3 mL	5 mg/mL	0.8 mL (80 units)	Not practical	Not practical

For most people, reconstituting at 5-10 mg/mL gives practical injection volumes between 20 and 80 units. You want to avoid volumes above 1 mL per injection for comfort.

Step 2: Clean the vial tops

Swab the rubber stopper on both the retatrutide vial and the bacteriostatic water vial with alcohol pads. Let them air-dry for a few seconds. This prevents bacteria from entering the vials.

Step 3: Draw the bacteriostatic water

Using a clean insulin syringe, draw your chosen volume of bacteriostatic water. Pull the plunger back to the correct mark before inserting the needle, then push the air into the BAC water vial to equalize pressure. Invert the vial and draw the water out slowly.

Step 4: Add water to the retatrutide vial

Insert the needle into the retatrutide vial at an angle so the water runs down the inside wall of the vial. Do not squirt the water directly onto the lyophilized powder. This can damage the peptide and create clumps that are hard to dissolve.

Release the water slowly. Let it trickle down the glass wall onto the powder.

Step 5: Let it dissolve

Set the vial on a flat surface and wait. The powder should dissolve within 2 to 5 minutes on its own. If it does not fully dissolve, gently roll the vial between your palms or tip it side to side slowly.

Do not shake the vial. Shaking creates bubbles and can damage peptide bonds, reducing potency. If you see foam or persistent bubbles, you shook it too hard. The solution should be clear and colorless when fully dissolved. If it is cloudy or has particles floating in it, do not use it.

Step 6: Store properly

Put the reconstituted vial in the refrigerator immediately. Keep it at 36-46°F (2-8°C). Do not freeze reconstituted retatrutide.

Label the vial with the date of reconstitution, total peptide content, water added, and concentration. Something like “10 mg + 2 mL BAC = 5 mg/mL, mixed 4/4/2026” prevents dosing errors later.

Dosing math made simple

The formula is: dose (mg) ÷ concentration (mg/mL) = injection volume (mL)

Convert mL to syringe units by multiplying by 100 (since 1 mL = 100 units on an insulin syringe).

Example: You have a 10 mg vial reconstituted with 2 mL of BAC water (concentration: 5 mg/mL). Your prescribed dose is 4 mg.

• 4 mg ÷ 5 mg/mL = 0.8 mL
• 0.8 mL × 100 = 80 units on the syringe

Pull the plunger to the 80 unit mark, and that is your 4 mg dose.

For help with dosing calculations, see our peptide dosage calculator. For the full week-by-week titration schedule for retatrutide, read our retatrutide dosage chart.

Storage and shelf life

Condition	Shelf life
Unreconstituted (lyophilized), room temp	Check label (typically 6-12 months)
Unreconstituted, refrigerated	Check label (typically 12-24 months)
Reconstituted with BAC water, refrigerated	Up to 28 days
Reconstituted with sterile water, refrigerated	Use within 24-48 hours

Bacteriostatic water contains 0.9% benzyl alcohol as a preservative, which is why reconstituted vials last up to 28 days. Sterile water has no preservative, so bacteria can grow quickly in a multi-use vial. Always use bacteriostatic water for peptides you will draw from multiple times. For more on this, read our bacteriostatic water guide.

Common mistakes to avoid

Adding too little water. If you add 0.5 mL to a 10 mg vial, the concentration is 20 mg/mL. Your 2 mg starting dose would be just 10 units on the syringe, which is very small and hard to measure accurately. Small measurement errors become big dosing errors at high concentrations.

Forgetting to label the vial. A week later, you will not remember exactly how much water you added. Write it on the vial or on tape stuck to it.

Using the wrong syringe. Standard 1 mL insulin syringes (U-100) are marked in 100 units. Some syringes are U-40 or U-50. Using the wrong syringe changes your measured volume. Stick with U-100 insulin syringes.

Storing at room temperature. Peptides degrade faster at room temperature. Refrigerate immediately after reconstitution and keep vials refrigerated between doses. Taking the vial out briefly for injection is fine, but do not leave it on the counter.

Reusing syringes. Each injection should use a new syringe and needle. Reuse dulls the needle (making injections more painful) and introduces bacteria. Insulin syringes are cheap. Use a new one every time.

Vial sizes and how many doses you get

Compounding pharmacies typically supply retatrutide in 5 mg, 10 mg, or 15 mg vials. The number of doses per vial depends on where you are in the titration schedule.

5 mg vial at 2 mg/week: 2.5 weeks of doses. You will need a new vial roughly every 2 weeks, or two vials per month during the initial titration phase.

10 mg vial at 4 mg/week: 2.5 weeks of doses. Again, about two vials per month.

10 mg vial at 8 mg/week: 1.25 weeks. You will go through nearly a vial per week at this dose level.

15 mg vial at 12 mg/week: 1.25 weeks. At the highest clinical trial dose, a single 15 mg vial barely covers one weekly injection.

This math matters for budgeting and ordering. Talk to your provider and pharmacy about packaging that matches your prescribed dose to minimize waste. Some compounding pharmacies offer larger vial sizes (20 mg or 30 mg) that make higher doses more practical.

For the complete retatrutide dosage chart showing the week-by-week titration schedule used in clinical trials, see our dosing guide.

Troubleshooting reconstitution issues

Sometimes things do not go perfectly. Here is how to handle common problems.

Powder sticks to the sides after adding water. This happens when water is added too quickly or sprayed directly onto the powder. Gently tip the vial at different angles to wash the water across the stuck powder. Wait 10 minutes. Most stuck particles dissolve on their own.

Small bubbles in the solution. Air bubbles from the mixing process are harmless and will rise to the top within a few minutes. Before drawing your dose, flick the vial gently to move bubbles to the top, then draw from below the bubble line.

You accidentally added too much or too little water. If you added too much, your concentration is lower than planned. Recalculate using the actual amount added: total mg in vial divided by actual mL of water equals your new concentration. If you added too little, you can add more BAC water to reach your target volume. Just adjust your total water calculation accordingly.

The powder will not dissolve after 15 minutes. Try gently rolling the vial between your palms for 30 seconds, then let it sit another 10 minutes. If particles remain after 30 minutes total, the peptide may be damaged. Contact your pharmacy.

Injection basics

After reconstitution, inject retatrutide subcutaneously (under the skin, not into muscle). Pinch a fold of skin at the injection site, insert the needle at a 45 to 90 degree angle, inject slowly, and hold for 5 to 10 seconds before withdrawing. Rotate injection sites between abdomen, outer thigh, and upper arm.

For complete injection technique, including how to deal with air bubbles, injection site reactions, and needle gauge selection, see our how to inject peptides guide and peptide injection sites page.

Frequently asked questions

What type of water do I use to reconstitute retatrutide?▼

Bacteriostatic water (BAC water) for multi-dose vials. It contains a preservative that prevents bacterial growth for up to 28 days. Do not use tap water, distilled water, or normal saline.

How much bacteriostatic water should I add?▼

That depends on your vial size and desired concentration. For a 10 mg vial, 2 mL of BAC water gives a concentration of 5 mg/mL, which produces practical injection volumes for most dose levels. See the reconstitution tables above for other vial sizes.

How long does reconstituted retatrutide last?▼

Up to 28 days when stored in the refrigerator at 36-46°F (2-8°C) and reconstituted with bacteriostatic water. Discard any remaining solution after 28 days.

Can I freeze reconstituted retatrutide?▼

No. Freezing and thawing can damage the peptide structure and reduce potency. Only freeze unreconstituted lyophilized powder if long-term storage is needed.

What if the solution is cloudy after mixing?▼

Do not inject it. A properly reconstituted peptide should be clear and colorless. Cloudiness or visible particles may indicate contamination, degradation, or improper mixing. Contact your pharmacy for a replacement.

Do I need to reconstitute prefilled pens?▼

No. If you receive retatrutide in a prefilled injection pen (as used in clinical trials), it comes ready to use. Reconstitution only applies to lyophilized powder in vials from compounding pharmacies.

References

1. Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-hormone-receptor agonist retatrutide for obesity — a phase 2 trial. N Engl J Med. 2023;389(6):514-526. doi:10.1056/NEJMoa2301972
2. United States Pharmacopeia. General Chapter <797> Pharmaceutical Compounding — Sterile Preparations. USP 2023.
3. Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss. Cell Metab. 2022;34(9):1234-1247.