GlossaryInjection & ReconstitutionInsulin Syringe
Injection & Reconstitution

Insulin Syringe

Also known as: U-100 syringe · insulin pin

An insulin syringe is a small, fine-needle syringe marked in insulin units; on the common U-100 type, 100 units equals 1 mL.

Insulin syringes are designed for subcutaneous injection of small volumes. Their barrels are marked in units rather than milliliters. A U-100 syringe is made to measure U-100 insulin, which contains 100 units per mL, so each unit mark on a U-100 syringe is 0.01 mL. Other scales exist, such as U-500 syringes for concentrated U-500 insulin, and the markings on one type do not measure correctly for another.

The barrel size sets the full scale: a 1 mL U-100 syringe holds 100 units, and a 0.3 mL U-100 syringe holds 30 units, with finer spacing between marks. The needle is short and thin. Insulin-injection guidance recommends the shortest needles, currently 6 mm for syringes, as the first choice, describing them as effective and less painful.

Why it matters

Peptide users use U-100 insulin syringes because they measure small volumes with reasonable resolution. But the unit marks only measure volume. Whether 10 units is the right dose depends entirely on the concentration of the solution in the vial.

Syringe mix-ups cause serious errors in medicine. The FDA-approved Humulin R U-500 label warns that measuring U-500 insulin with a U-100 syringe can give a five-times overdose. The same logic applies to peptides: the syringe type and the solution's concentration must both be known before a unit number means anything. Used syringes are also a hazard to other people: insulin-injection guidance notes that improper disposal of sharps risks infection with blood-borne pathogens, so used syringes belong in a puncture-resistant sharps container. Treat every syringe as single-use, because a used needle is no longer sterile and can contaminate a multi-dose vial.

How to read one

  1. Confirm the syringe is marked U-100 on the packaging or barrel.
  2. Note the barrel size (for example 0.3, 0.5 or 1 mL) and what each small line represents.
  3. Check the needle length on the package; injection guidance treats the shortest needles as the first choice for subcutaneous injection.
  4. Convert your dose to units with your vial's concentration before you draw.
  5. Use a new sterile syringe for every injection and dispose of it in a sharps container.

mL = units ÷ 100 (U-100 syringe)

Worked example

On a U-100 syringe, 25 units is 25 ÷ 100 = 0.25 mL. On a 0.3 mL barrel, the full scale is 0.3 × 100 = 30 units.

Common mistakes

  • Using a syringe without checking that it is U-100.
  • Treating unit marks as a dose instead of a volume.
  • Reusing a syringe; the Humulin R U-500 instructions, for example, label their syringe for a single injection only.
  • Using a long needle and injecting straight in on a lean area, which can reach muscle.
  • Copying someone else's unit number without matching their concentration.

Sources

  1. Prescribing informationHumulin R U-500 (insulin human) injection - prescribing information and patient information. DailyMed, U.S. National Library of Medicine, 2026.
  2. Prescribing informationHumulin R U-100 (insulin human) injection - prescribing information. DailyMed, U.S. National Library of Medicine, 2023.
  3. GuidelineNew Insulin Delivery Recommendations. Mayo Clin Proc, 2016. PMID 27594187.

Page updated . How we grade evidence: methodology.

Related Terms

Units (Insulin Syringe)

Units on an insulin syringe are volume markings; on a U-100 syringe, 1 unit = 0.01 mL and 100 units = 1 mL. They are not a dose by themselves.

Subcutaneous (SubQ) Injection

A subcutaneous (SubQ) injection delivers a drug into the fat layer between the skin and the muscle, where it is absorbed gradually.

Drawing Dose

Drawing a dose means pulling a measured volume of solution from a vial into a syringe, removing air bubbles, and setting it to the exact mark.

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