Guides / Mixing and syringes

U-100 vs U-40 vs U-500 syringes, and needle sizes

The number after the U tells you how much liquid each unit line holds, so a syringe from the wrong scale gives the wrong dose even when you read it perfectly.

By PeptideTracker editorial team · Updated November 8, 2026 · 15 sources

[00] Short answer

  1. The U number is units per mL. The 10 line holds 0.1 mL on a U-100 syringe, 0.25 mL on U-40 and 0.02 mL on U-500.
  2. Mixing up scales puts the dose off by 2.5 times (U-40 vs U-100) or 5 times (U-500 vs U-100). In one FDA-described case, "70 units" on a U-100 syringe from a U-500 pen was 350 units5.
  3. Gauge: the higher the number, the thinner the needle7. Ozempic and Wegovy FlexTouch pens come with 32G 4 mm needles11–12.
  4. Skin at injection sites averages 1.9 to 2.4 mm thick. A 4 to 5 mm needle put in straight reaches the fat layer with minimal risk of hitting muscle in virtually all adults8.

What's the difference between U-100, U-40 and U-500 syringes?

A U-100 syringe has 100 unit marks per mL, a U-40 has 40 and a U-500 has 500. So the same printed number holds a different amount of liquid on each. Standard insulin, and the math on this site, use U-1001.

How much liquid each line holds, by syringe scale
LineU-100U-40U-500
The 5 line0.05 mL0.125 mL0.01 mL
The 10 line0.1 mL0.25 mL0.02 mL
The 20 line0.2 mL0.5 mL0.04 mL
The 50 line0.5 mL1.25 mL0.1 mL

U-100 and U-40 volumes are computed with the PeptideTracker dose math. U-500 is units ÷ 500.

  • U-100 is the standard insulin syringe. U-100 insulin has 100 units in each mL1.
  • U-40 is for some pet insulins. Vetsulin is 40 units per mL, and its owner sheet says any syringe other than U-40 will give the wrong dose3.
  • U-500 syringes were made for Humulin R U-500 insulin. embecta's has a green U-500 symbol and a green needle shield2. The current US label (July 2026) lists Humulin R U-500 only as a KwikPen and says never to draw it from the pen into any syringe4.
[Tool]Convert syringe lines to mL0.3, 0.5 and 1 mL U-100 syringes, with a drawing

Why is using the wrong syringe scale dangerous?

Because you can read the syringe correctly and still get the wrong dose. Drawing to the 10 line on a U-40 syringe gives 0.25 mL, which is 2.5 times the liquid of the U-100 10 line. A U-500 10 line gives one fifth.

  • A patient left hospital with a U-500 insulin pen and no pen needles. She used a U-100 syringe to draw "70 units" from the pen and got 350 units. Her children found her unresponsive, and she recovered after emergency care5.
  • The Humulin R U-500 label says moving insulin from the KwikPen into any syringe can cause an overdose and severe low blood sugar4.
  • With compounded semaglutide, the FDA has reports of people told to draw 5 units on a U-100 syringe who drew 50 instead6. See units vs mg vs mL.

In numbers: 0.7 mL of U-500 insulin is 0.7 × 500 = 350 units, five times what the U-100 scale showed.

What do needle gauge numbers mean?

Gauge is the needle's thickness, and the scale runs backward: the higher the number, the thinner the needle7. A 32G needle is thinner than a 31G one. Insulin syringe needles go down to 31G, and pen needles as thin as 34G7.

  • Studies found pain more often with thicker needles7.
  • Very thin needles can need more force to push the dose through, and can bend or leak more easily7.
  • Mixing and injecting can use different needles. The EGRIFTA SV kit has 1-inch 20-gauge needles for mixing and half-inch 30-gauge needles for injecting15.

What needle length do you need for a subcutaneous injection?

Short needles reach the fat under the skin in almost everyone. In a study of 388 US adults with diabetes, skin averaged 1.9 to 2.4 mm thick and the fat layer 10.4 to 15.4 mm, depending on the site. Needles of 4 to 5 mm put in at 90 degrees reached fat with minimal risk of muscle in virtually all adults8.

Average skin and fat thickness at injection sites, 388 adults with diabetes (Gibney 2010)
SiteSkinFat layer under skin
Upper arm2.2 mm10.8 mm
Thigh1.9 mm10.4 mm
Abdomen2.2 mm13.9 mm
Buttocks2.4 mm15.4 mm

Women had 5.1 mm more fat on average. Needles 8 mm or longer, put in straight, may often reach muscle in the limbs of men and of people with a BMI under 258.

Expert recommendations from 2016 call the shortest needles, 4 mm for pens and 6 mm for syringes, the first choice for all patients9. embecta's insulin syringes come with 6, 8 or 12.7 mm needles2.

Two caveats. These studies were in people using insulin, not GLP-1 medicines, and the authors of a 4 mm needle trial said trials in GLP-1 users were still needed10. Several of the papers were written or funded by a needle maker, BD7–10.

Which needles go with GLP-1 pens and vials?

It depends on the device. Ozempic and Wegovy FlexTouch pens include 32G 4 mm needles. Mounjaro and Zepbound KwikPens need pen needles bought separately, and Zepbound vials need a syringe and needle your provider recommends11–14.

Needles for GLP-1 pens and vials (US Instructions for Use)
DeviceNeedleNotes from the label
Ozempic penNovoFine Plus 32G 4 mm, included11Other compatible needles up to 8 mm can be used11
Wegovy FlexTouch penNovoFine Plus 32G 4 mm, included12New needle each injection12
Mounjaro or Zepbound KwikPenKwikPen-compatible pen needle, not included13–14New needle each time, to help prevent infection and blocked needles13
Zepbound single-dose penBuilt in, under a gray base cap14Nothing to attach14
Zepbound vialSyringe and needle not included14For example a 1 mL syringe, as your provider recommends14

Pen needles have two caps. The Ozempic instructions say that if you forget to remove both, you won't inject any medicine11. The FDA lists a missed inner cap among common pen errors5.

Questions

Can I use a U-100 syringe for U-40 insulin?

Not without a conversion, and pet insulin labels like Vetsulin's say to use U-40 syringes only. The 10 line holds 0.1 mL on U-100 and 0.25 mL on U-40.

Is a lower or higher gauge needle thinner?

Higher. A 32G needle is thinner than a 31G needle, which is thinner than a 30G needle.

What size needle comes with Ozempic?

NovoFine Plus 32G 4 mm needles are included with the Ozempic pen. The label says other compatible needles up to 8 mm can also be used.

Is a 4 mm needle long enough?

For under-the-skin injections, a study of 388 adults found skin is about 1.9 to 2.4 mm thick, and 4 to 5 mm needles put in straight reached the fat layer in virtually all adults. Those were insulin users.

Can I reuse a pen needle?

The Ozempic and Mounjaro instructions say to use a new needle for each injection. The Mounjaro instructions say this helps prevent infections and blocked needles.

What color is a U-500 syringe?

embecta's U-500 syringe has a green U-500 symbol and a green needle shield. The current US Humulin R U-500 label lists only a pen, and says not to draw from it into any syringe.

Sources

Checked against MedlinePlus, embecta syringe information, the Vetsulin owner sheet, the Humulin R U-500 US label (2026), FDA dosing-error reports, Ozempic, Wegovy, Mounjaro, Zepbound and EGRIFTA SV labels, and PubMed studies on skin thickness and needle length. How we check

  1. 1MedlinePlus (US National Library of Medicine), Giving an insulin injection. reviewed 21 Jul 2024
  2. 2embecta, Insulin syringes with Ultra-Fine needle (US product information). needle lengths, U-500 syringe; accessed 8 Nov 2026
  3. 3Vetsulin (porcine insulin zinc suspension) owner information sheet, Animal Drugs @ FDA. U-40 syringes only
  4. 4Humulin R U-500 (insulin human) KwikPen, US prescribing information (DailyMed). Eli Lilly, Jul 2026; sections 2.3 and 3
  5. 5FDA Drug Topics: How to Avoid Medication Errors with Pen Injectors (FDA and ISMP presentation). 2023; misuse of concentrated insulins with U-100 syringe; pen errors table
  6. 6FDA, FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. content current as of 26 Jul 2024
  7. 7Heinemann L et al. Needle technology for insulin administration: a century of innovation. J Diabetes Sci Technol 2023;17(2):449-457. PMID 34889142; funded by BD, two authors BD employees
  8. 8Gibney MA et al. Skin and subcutaneous adipose layer thickness in adults with diabetes at sites used for insulin injections. Curr Med Res Opin 2010;26(6):1519-30. PMID 20429833; authors from BD
  9. 9Frid AH et al. New insulin delivery recommendations. Mayo Clin Proc 2016;91(9):1231-55. PMID 27594187; FITTER workshop, several authors from BD
  10. 10Hirsch LJ et al. Comparative glycemic control, safety and patient ratings for a new 4 mm x 32G insulin pen needle in adults with diabetes. Curr Med Res Opin 2010;26(6):1531-41. PMID 20429832; limitations section
  11. 11Ozempic (semaglutide) injection, US prescribing information and Instructions for Use. Novo Nordisk, revised 05/2026
  12. 12Wegovy (semaglutide) injection and tablets, US prescribing information and Instructions for Use. Novo Nordisk, revised 06/2026; FlexTouch Instructions for Use
  13. 13Mounjaro (tirzepatide) injection, US prescribing information and Instructions for Use (DailyMed). Eli Lilly, Aug 2026; KwikPen Instructions for Use
  14. 14Zepbound (tirzepatide) injection, US prescribing information and Instructions for Use (DailyMed). Eli Lilly, Aug 2026; pen, vial and KwikPen Instructions for Use
  15. 15EGRIFTA SV (tesamorelin for injection) US prescribing information (DailyMed). Theratechnologies, Jul 2026; injection box contents

This guide explains what labels and published studies say. It isn't medical advice and doesn't replace your prescriber or pharmacist.

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