IU vs mg: When You Can and Cannot Convert
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[Glossary · Mixing and measuring]

IU vs mg

Also called IU to mg, units vs mg, international units vs milligrams · Reviewed

Definition: IU vs mg is the difference between measuring a drug by its activity (international units, IU) and by its weight (milligrams, mg). The two convert only when a reference standard fixes the ratio for that one substance. If no ratio has been set for a drug, there is no correct IU-to-mg number to use.

Three different kinds of "units"

The word "unit" can mean three different things
What you seeWhat it measuresLabel example
IU or USP unitsBiological activity, against a reference standardHumulin R: 100 units of insulin per mL
mg or mcgWeightForteo: 250 mcg of teriparatide per mL
Syringe unitsVolume: each mark on a U-100 syringe is 0.01 mLHumulin R carton: use only with a U-100 syringe

Only the first is an IU. Syringe marks are volume, and they equal insulin units only with insulin made to match the syringe. See insulin syringe units.

When a conversion exists

For somatropin (HGH), the First (1995) and Second (2001) International Standards both set 3.0 IU per mg. See international unit for how such a standard is made.

Omnitrope 5.8 mg vial in IU

  1. Whole vial: 5.8 mg × 3 IU/mg = 17.4 IU.
  2. The label says the mixed vial is 5 mg/mL. In IU: 5 × 3 = 15 IU/mL.
  3. Check: 0.1 mL × 15 IU/mL = 1.5 IU, and 0.1 mL × 5 mg/mL = 0.5 mg, which is 1.5 IU at 3 IU/mg.

When it does not

The 3 IU per mg ratio is for somatropin only. Forteo and Wegovy have no IU at all; their labels give strength by weight, in mcg and mg. In 2024 FDA reported dosing errors with compounded semaglutide where confusion between mL, mg and "units" played a part. Semaglutide is measured in mg; the "units" in those cases were syringe marks. Keep the unit next to every number you log.

Read more on Dose Track: HGH (somatropin) guide · GLP-1 units to mg charts · Peptide reconstitution calculator

General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
International unit (IU)An international unit (IU) measures a drug by its biological activity, compared with an agreed reference… mcg vs mg (micrograms and milligrams)mg (milligram) and mcg (microgram) are units of weight. One milligram is a thousandth of a gram; one… Insulin syringe unitsThe units on an insulin syringe are marks of volume. On a U-100 syringe, 1 unit = 0.01 mL; on a U-40… Concentration (mg/mL)Concentration is how much drug is in each unit of volume, usually written as mg/mL, mcg/mL or units/mL. It… mg/mL (milligrams per milliliter)mg/mL (milligrams per milliliter) is a strength: how many milligrams of drug are in each milliliter of…

Frequently asked questions

Can you convert IU to mg for any drug?+
No. A conversion only exists when a reference standard sets the ratio for that substance. For somatropin it is 3.0 IU per mg; that number does not apply to other drugs.
How many IU are in a 5.8 mg somatropin vial?+
At 3.0 IU per mg, 5.8 mg is 17.4 IU.
Are the units on a compounded semaglutide syringe IU?+
No. Semaglutide is measured in mg. The marks on an insulin syringe measure volume, and FDA has linked confusion between mL, mg and units to dosing errors.

Sources

  1. Bristow et al. Growth Regul 1995 (First International Standard for somatropin, 3.0 IU/mg). PMID 7580864
  2. Bristow and Jespersen. Biologicals 2001 (Second International Standard for somatropin, 3.0 IU/mg). PMID 11580214
  3. Omnitrope (somatropin) US prescribing information, DailyMed
  4. Humulin R U-100 (insulin human) US prescribing information, DailyMed
  5. Forteo (teriparatide injection) US prescribing information, DailyMed
  6. Wegovy (semaglutide) US prescribing information, DailyMed
  7. FDA: Dosing errors associated with compounded injectable semaglutide products (2024)

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