Understanding dosing language: mcg, mg, IU, and units

Peptide dosing uses a handful of measurement terms that sound more technical than they are. Once you understand what each unit represents, dosing conversations become clearer.

Milligrams and micrograms: weight-based measurement

Most peptides are dosed by weight. You're measuring the actual mass of the peptide.

Milligram (mg) is one-thousandth of a gram — the standard unit for most peptides. Microgram (mcg) is one-thousandth of a milligram, used for peptides effective at very small doses.

The relationship is straightforward:

• 1 mg = 1,000 mcg
• 0.1 mg = 100 mcg
• 0.01 mg = 10 mcg

Mistakes happen when people confuse the two. A dose of 250 mcg is very different from 250 mg.

International units: a biological standard

Some peptides are measured in international units (IU), which reflect biological activity rather than mass. IU is used when the same peptide can exist in slightly different forms or potencies.

You'll see IU most often with hCG, hGH, and some forms of insulin. The conversion between IU and mg varies by compound — you can't convert without knowing the specific peptide and its established standard.

"Units" without the "international"

Insulin uses units (U), based on biological effect. Most insulin is formulated at U-100 concentration (100 units per milliliter). Insulin has its own syringe types marked in units rather than milliliters, which prevents dosing errors.

How the reconstitution math works

The concentration after reconstitution determines how much liquid you inject.

Example: a 5 mg vial + 2 mL of bacteriostatic water = 2.5 mg/mL. If your target dose is 250 mcg (0.25 mg), you'd inject 0.1 mL.

There's no single "right" dilution — just make sure you know the final concentration so you can calculate accurately.

Why this matters

Dosing precision matters with peptides. Understanding the vocabulary reduces errors and builds confidence. When in doubt, ask your practitioner to clarify — good ones welcome the question.