What Is an International Unit (IU)? IU vs mg
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[Glossary · Mixing and measuring]

International unit (IU)

Also called IU, USP unit, units of activity, IU to mg · Reviewed

Definition: An international unit (IU) measures a drug by its biological activity, compared with an agreed reference standard, rather than by weight. It is used for hormones and other biologics where activity matters more than mass, such as somatropin (HGH) and hCG. The IU-to-mg ratio is different for every substance.

How an IU is defined

A reference preparation is made and studied by many labs, and its activity is assigned a number of units. For somatropin, the First International Standard (1995) and the Second (2001) both set a specific activity of 3.0 IU per mg. So for somatropin, 1 mg = 3 IU.

That ratio is for somatropin only. hCG labels such as Novarel and Pregnyl state strength in USP units (5,000 or 10,000 per vial) and do not give a mg figure at all. Insulin is labeled in units too; Humulin R is 100 units/mL.

Worked examples

Somatropin: IU to mg

  1. Ratio from the international standard: 3 IU = 1 mg.
  2. 4 IU ÷ 3 = 1.33 mg.
  3. The other way: 2 mg × 3 = 6 IU.

hCG: units to volume

  1. Novarel 10,000 units mixed with 10 mL: 10,000 ÷ 10 = 1,000 units/mL.
  2. Volume for 500 units: 500 ÷ 1,000 = 0.5 mL.
  3. On a U-100 syringe, 0.5 mL is the 50 mark. The 50 marks are volume, not 50 IU.

Common mix-ups

  • Using one ratio for every drug. 3 IU per mg is somatropin. It does not apply to hCG, insulin or anything else.
  • Reading syringe "units" as IU. The marks on an insulin syringe are volume. They equal insulin units only with insulin of the matching strength. See insulin syringe units.
  • Dropping the unit in your log. Record IU, mg or mL every time.

Read more on Dose Track: HGH (somatropin) guide · hCG guide · 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.
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Frequently asked questions

How many IU are in 1 mg of HGH?+
For somatropin, the international reference standards set 3.0 IU per mg, so 1 mg is 3 IU.
Is an IU the same for every drug?+
No. Each substance has its own reference standard, so the IU-to-mg ratio differs from drug to drug.
Are IU and the units on an insulin syringe the same thing?+
No. Syringe marks measure volume (0.01 mL each on a U-100 syringe). They match insulin units only with U-100 insulin.

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. Novarel (chorionic gonadotropin) US label, DailyMed
  4. Pregnyl (chorionic gonadotropin) US label, DailyMed
  5. Humulin R U-100 (insulin human) US prescribing information, DailyMed

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