IU vs mg: per-drug conversions explained
International Units (IU) appear on insulin, growth hormone, gonadotropins, and a few legacy peptides. The unit isn't a fixed mass — it's defined by biological activity in a standardized assay. Every drug has its own IU-to-mg conversion. Mixing them up produces predictable dosing errors.
Why IU exists at all
Before pure synthetic forms of insulin, growth hormone, and gonadotropins were available, drugs were extracted from animal or human tissue with variable potency per gram. "International Units" were defined by reference standards held by WHO and standardized through in-vivo or in-vitro bioassay — biological activity per dose unit, regardless of actual molecular content.
As synthetic recombinant forms replaced extracted ones, IU labeling persisted by convention. The IU-to-mg conversion became a fixed quantity for each well-characterized drug.
The conversions to know
| Drug | 1 IU equals | Common dose |
|---|---|---|
| Insulin (human) | 0.0347 mg (34.7 mcg) | 10–40 IU daily for type 2; varies widely |
| Human growth hormone (HGH) | 0.333 mg (333 mcg) | Adult deficiency: 0.2–0.5 mg/day = 0.6–1.5 IU/day |
| FSH (Follistim, Gonal-F) | ~0.088 mg per IU (preparation-dependent) | 75–150 IU per cycle dose |
| HCG | ~0.001 mg per IU | 1,500–10,000 IU per dose for fertility uses |
These conversion factors are specific. Reading "30 IU growth hormone" as "30 IU insulin" gives wildly different absolute drug amounts.
Common IU traps
Trap 1: Generalizing the insulin conversion
"1 IU = some fixed milligram amount" — only true for one drug at a time. The actual mass for 1 IU of growth hormone is 10× the mass for 1 IU of insulin.
Trap 2: HGH labeled in IU when manuals use mg
Many HGH vials are labeled in IU. Most clinical dosing recommendations (in adult deficiency, especially recent ones) use mg. A patient with a "0.4 mg/day" prescription and a "12 IU" vial needs to know that 12 IU = 4 mg total — so 0.4 mg = 1.2 IU per dose. Drawing "0.4" of anything from a 12 IU vial without conversion is incorrect.
Trap 3: HGH "IU" preparations vary by brand
HGH conversion is 0.333 mg/IU for the WHO standard. Some HGH brands historically used slightly different conversions (3 IU = 1 mg vs 2.7 IU = 1 mg). Newer products are more consistent, but legacy products and grey-market HGH may use different conversions. Check the package insert.
Trap 4: Pen syringes mismatched to drug
U-100 insulin syringes are calibrated assuming 100 IU = 1 mL. Drawing HGH from a vial reconstituted at 1 IU/mL into a U-100 syringe means "1 unit" on the syringe = 0.01 mL = 0.01 IU. A wildly different scale. Use the right syringe for the drug.
Insulin specifics
Insulin is the most common IU drug. The 0.0347 mg conversion is for human insulin and most insulin analogs:
- Regular insulin (Humulin R, Novolin R): 0.0347 mg/IU.
- Rapid-acting (Humalog, NovoLog, Apidra): 0.0347 mg/IU.
- Long-acting (Lantus, Tresiba, Toujeo): 0.0347 mg/IU.
- U-500 (Humulin R U-500): 5× concentrated. 1 IU is still 0.0347 mg — but per mL of solution, there are 500 IU instead of 100.
For practical purposes, insulin dosing is in IU on the syringe and in IU on the prescription. The mg conversion rarely appears in clinical practice; it's mostly relevant in research and rare cross-conversion situations.
Growth hormone specifics
HGH is the IU drug where the conversion matters most because doses are typically prescribed in mg but vials are sometimes labeled in IU:
- 0.333 mg = 1 IU is the WHO standard.
- Common dosing: 0.2–0.5 mg/day in adult deficiency; higher in pediatric.
- Multi-dose vials: often 12 IU (4 mg) or 36 IU (12 mg) per vial.
Reconstitution math: a 4 mg vial reconstituted with 1 mL of diluent gives 4 mg/mL = 12 IU/mL. A 0.4 mg dose = 0.1 mL = 1.2 IU = 10 units on a U-100 syringe.
The discipline
- Read the label's unit explicitly. "IU" and "mg" are not interchangeable. Don't assume one because you're used to the other.
- Apply the per-drug conversion if needed. Insulin: 0.0347 mg/IU. HGH: 0.333 mg/IU. Other peptides: check the COA.
- Use the right syringe. U-100 for insulin, U-100 for HGH if the concentration is appropriate, otherwise a 1 mL syringe with 0.01 mL graduations.
- Document the unit in your dose log. "0.5 mg of X" leaves no ambiguity; "5 of X" is dangerous.
FAQ
Why do some HGH labels use "IU" and others use "mg"?
Historical convention. European and some legacy U.S. products kept IU; newer products and clinical guidelines tend toward mg. Both refer to the same drug; the conversion is fixed.
Is the IU-to-mg conversion ever updated?
Rarely. The WHO references are stable; the conversions in this post are the long-standing values. Branded HGH has occasionally introduced small per-product variations.
For research-use peptides, what unit do I use?
Most research-use peptides (BPC-157, TB-500, GHK-Cu) are labeled in mg, not IU, because they aren't standardized by bioassay. Stick with mg and use the concentration math.
Does anabolic-steroid context use IU?
Anabolic steroids are mass-based (mg). HGH and HCG in performance contexts use IU. The mixed convention is part of why dosing errors happen with these stacks.
Related reading
Convert IU and mg per drug
Peptide Protocol stores the per-drug conversion and refuses to compute a dose if the unit doesn't match the drug's spec.
Get the iPhone app →Informational and educational only. Not medical advice. Consult a licensed clinician before starting, changing, or stopping any peptide protocol. Mentions of investigational, compounded, or research-use peptides are for informational purposes; many such substances are not FDA-approved for human use.