- Your dose is prescribed in milligrams (mg); insulin syringes are marked in units. On a standard U-100 syringe, 100 units is 1 millilitre.
- Units = dose in mg ÷ vial strength in mg/mL × 100. The same mg dose is a different number of units in a different strength of vial.
- Always use the strength printed on your own vial's label, and ask your provider to confirm your dose in both mg and units.
- Keep medication refrigerated, throw a multi-dose vial away 28 days after first use, and do not inject anything that arrived warm until the pharmacy confirms it is safe.
Why units and milligrams get confused
Brand-name pens deliver a set dose when you click or press them, so there is nothing to measure. Compounded semaglutide and tirzepatide, and Zepbound® vials, come as liquid that you draw up with a syringe. Your prescription says how many milligrams to take, the vial says how many milligrams are in each millilitre, and the syringe is marked in units. Three different numbers, and it is easy to use the wrong one.
FDA has received reports of people taking five to twenty times their intended dose this way, some of whom needed hospital care, and of clinicians miscalculating doses too 1. Its advice is that prescribers supply the right syringes and show patients exactly how much to draw.
The formula
A U-100 insulin syringe holds 100 units in 1 millilitre, so each unit is 0.01 mL. Your vial strength is printed on the pharmacy label, for example "2.5 mg/mL".
Worked examples for two common vial strengths. Yours may differ; always use your own label.
| Semaglutide dose | 2.5 mg/mL vial | 5 mg/mL vial |
|---|---|---|
| 0.25 mg | 10 units (0.10 mL) | 5 units (0.05 mL) |
| 0.5 mg | 20 units (0.20 mL) | 10 units (0.10 mL) |
| 1 mg | 40 units (0.40 mL) | 20 units (0.20 mL) |
| 1.7 mg | 68 units (0.68 mL) | 34 units (0.34 mL) |
| 2.4 mg | 96 units (0.96 mL) | 48 units (0.48 mL) |
| Tirzepatide dose | 10 mg/mL vial | 20 mg/mL vial |
|---|---|---|
| 2.5 mg | 25 units (0.25 mL) | 12.5 units (0.125 mL) |
| 5 mg | 50 units (0.50 mL) | 25 units (0.25 mL) |
| 7.5 mg | 75 units (0.75 mL) | 37.5 units (0.375 mL) |
| 10 mg | 100 units (1.00 mL) | 50 units (0.50 mL) |
| 15 mg | Too large for one 1 mL syringe | 75 units (0.75 mL) |
Calculated with the formula above. These are examples of the arithmetic, not dosing advice; your clinician sets your dose.
A clinician demonstration of drawing a dose from a vial:
Five checks before every new vial
- Read the strength on the label. If you have switched pharmacy or provider, it may have changed, and the same milligram dose will be a different number of units.
- Confirm the active ingredient is semaglutide or tirzepatide, not a salt form such as semaglutide sodium or acetate, which FDA says should not be compounded 2.
- Ask your provider to state your dose in both mg and units, in writing.
- Check the syringe is a U-100 insulin syringe if your instructions are in units, and that it is big enough for your dose.
- If a dose suddenly feels much stronger, stop and recheck the arithmetic before your next injection.
Storing your medication
- Compounded vials: keep them in the fridge unless the pharmacy label says otherwise, and throw a multi-dose vial away 28 days after you first use it, even if medicine is left 2.
- Zepbound® pens and vials: refrigerate at 36°F to 46°F. If needed, they can be kept unrefrigerated up to 86°F for up to 21 days 4.
- Wegovy® pens: refrigerate at 36°F to 46°F. If needed, before the cap is removed, a pen can be kept between 46°F and 86°F for up to 28 days 3.
- Never freeze a GLP-1, and do not use one that has been frozen. Keep it out of direct heat and light.
If your delivery arrives warm
Warm or late cold-chain deliveries are a common complaint across many programs, and support teams do not always give consistent answers. FDA's advice is plain: do not use an injectable that arrives warm 2. In practice:
- Do not inject it yet. Put it in the fridge while you check.
- Photograph the package, the ice packs and the vial, and note the date and time it arrived.
- Contact the pharmacy or your provider and ask in writing whether it is safe to use or will be replaced.
- If you will miss a dose, tell your clinician how long it has been; after a gap, the dose may need to be restarted lower.
Disposing of needles
Put used needles and syringes straight into an FDA-cleared sharps container. If you do not have one, a heavy-duty plastic household container with a tight lid will do. Never put loose needles in household waste or recycling, and check your state or local rules for how to dispose of a full container 5.
Questions and answers
How many units is 0.25 mg of semaglutide?
It depends on the vial's strength. In a 2.5 mg/mL vial, 0.25 mg is 0.1 mL, which is 10 units on a U-100 syringe. In a 5 mg/mL vial it is 5 units. Always use the strength on your own label.
How many units is 2.5 mg of tirzepatide?
In a 10 mg/mL vial, 2.5 mg is 0.25 mL, or 25 units on a U-100 syringe. In a 20 mg/mL vial it is 12.5 units. Check your label.
What syringe should I use?
Use the syringes your provider or pharmacy supplies, and check they are U-100 insulin syringes if your instructions are in units. FDA has asked prescribers to supply correctly sized syringes and show patients how to measure.
My medication arrived warm. Is it safe?
Do not inject it until you have checked. Contact the pharmacy or your provider with photos of the package and ice packs, and ask whether it is usable or will be replaced. FDA advises not to use an injectable that arrives warm.
How long does a semaglutide vial last once opened?
FDA's guidance is to throw a multi-dose vial away 28 days after first use, even if there is medicine left, unless your pharmacy's label says otherwise.
Can I keep my Zepbound or Wegovy pen out of the fridge?
For a limited time. The labels allow Zepbound® pens and vials to be kept unrefrigerated up to 86°F for up to 21 days, and Wegovy® pens between 46°F and 86°F for up to 28 days. Never freeze them.
How do I get rid of used needles?
Put them straight into an FDA-cleared sharps container, or a heavy-duty plastic household container if you do not have one, and follow your local rules for disposal.
Keep reading
Sources
- FDA: dosing errors associated with compounded semaglutide (July 26, 2024)
- FDA: concerns with unapproved GLP-1 drugs used for weight loss
- Wegovy® prescribing information and instructions for use
- Zepbound® prescribing information and instructions for use
- FDA: safely using sharps (needles and syringes) at home, at work and on travel
How we write our guides. We read the manufacturers' prescribing information, FDA and other official sources, published trials and each provider's own pricing and policies, and link to them so you can check our work. Figures were correct when we last checked them on October 19, 2026; prices and rules change often, so confirm them before you act. Medical disclaimer. This guide is general information, not medical advice, and no one at ProviderCompare is your clinician. Talk to a licensed healthcare professional before starting, changing or stopping any medication. Compounded GLP-1 medications are not FDA-approved and do not undergo FDA review for safety, effectiveness or quality. Brand names are trademarks of their owners, used for identification only.