Syringes and needles
Which insulin syringe fits a dose, why U-40 syringes cause overdoses, what needle length and gauge mean, and why two drugs shouldn’t share a syringe.
Insulin syringes measure in units
Peptide doses are usually measured with U-100 insulin syringes. U-100 means 100 units fill 1 mL, so 1 unit is 0.01 mL on any U-100 syringe, whatever is in it. The units measure volume; how much peptide that volume holds depends on how the vial was mixed.
| Syringe | Holds | Each line | Best for |
|---|---|---|---|
| 0.3 mL | 30 units | 1 unit | Doses up to 30 units; the easiest to read |
| 0.5 mL | 50 units | 1 unit | Doses of 30 to 50 units |
| 1 mL | 100 units | 2 units | Doses over 50 units; odd numbers fall between lines |
Pick the smallest syringe that holds the whole dose. For adding water to a vial, a 1 to 3 mL syringe marked in mL is easier. The reconstitution guide shows how to read each one.
U-40 syringes: a 2.5-times mistake
U-40 syringes are made for insulins with 40 units per mL, such as some pet insulins, whose labels say to use nothing else. Each U-40 unit is 0.025 mL, so a dose worked out for a U-100 syringe and drawn on a U-40 one is 2.5 times too large.
| Mark on the syringe | U-100 syringe | U-40 syringe |
|---|---|---|
| 10 units | 0.10 mL | 0.25 mL |
| 20 units | 0.20 mL | 0.50 mL |
| 40 units | 0.40 mL | 1.00 mL |
Needle length and thickness
Under the skin, short needles are enough. In ultrasound measurements of 388 adults with diabetes, skin was about 2 mm thick at the usual injection sites, with an average of 10 to 14 mm of fat beneath it. An international expert panel recommends the shortest needles, 4 mm on pens and 6 mm on syringes, as the first choice for every adult: they’re safe, effective and less painful, and injections into muscle should be avoided.
A needle’s gauge is its thickness: the higher the number, the thinner the needle. Injections into muscle need longer needles and should be taught by a clinician.
Two drugs in one syringe
Don’t mix approved drugs yourself. Ozempic’s and Mounjaro’s labels say to give them separately from insulin and never to mix them; both can go in the same area, but not right next to each other. Where two drugs do share a pen, as in the insulin and GLP-1 combinations Xultophy and Soliqua, the maker combined and tested them as one product. No label covers mixing research peptides in one syringe, and we know of no published study of whether they stay stable together.
A blend is different: it’s sold already mixed, and every unit holds all of its peptides in a fixed ratio. Blends are listed in the library.
One needle, one time
- Use a new needle and syringe for every injection, and a new one each time you enter a vial.
- Never share needles, syringes or vials, even with family. Shared needles can pass on hepatitis B, hepatitis C and HIV.
- Wipe the vial’s stopper with alcohol and let it dry before each puncture.
- Put used sharps straight into a sharps container; the injection guide covers disposal.
- If someone is stuck by a used needle, wash the spot with soap and water and get care right away. See Is something wrong?
Sources: Frid et al., New insulin delivery recommendations, Mayo Clin Proc 2016, PMID 27594187; Gibney et al., Curr Med Res Opin 2010, PMID 20429833; FDA-approved pet insulin labeling (ProZinc, 2019); Ozempic, Mounjaro, Xultophy and Soliqua prescribing information (2025–2026); CDC injection safety checklist; MedlinePlus, After an exposure to sharps or body fluids (reviewed October 2025); FDA, Safely using sharps at home.