Why Units Matter
Measurement units in peptide research can be confusing. Different studies may express the same compound in different units, making comparisons difficult without understanding the underlying measurements.
This guide explains the most common units you'll encounter when reading peptide research literature.
Mass-Based Units
Milligrams (mg)
A milligram is one-thousandth of a gram (1 mg = 0.001 g). This is a relatively large unit for many peptides and is often used for bulk quantities or less potent compounds.
Micrograms (mcg or μg)
A microgram is one-millionth of a gram (1 mcg = 0.001 mg = 0.000001 g). This is the most common unit for peptide dosages in research, as many peptides are active at microgram levels.
Note: "mcg" and "μg" are equivalent. "mcg" is often used in contexts where the Greek letter μ might cause display issues.
Conversion
The relationship between these units:
- 1 mg = 1,000 mcg
- 1 mcg = 0.001 mg
- 1 g = 1,000 mg = 1,000,000 mcg
International Units (IU)
International Units are different from mass-based units. An IU measures biological activity rather than mass. The conversion between IU and mass units varies by compound.
Why IU Exists
Some compounds vary in potency between batches or sources. IU standardizes measurements based on biological effect rather than mass, allowing for more consistent comparisons.
Conversion Challenges
There is no universal IU-to-mg conversion because each compound has its own definition of what constitutes one IU. For example:
- Growth hormone: approximately 3 IU = 1 mg (varies by formulation)
- Insulin: standardized differently
- Each peptide with an IU standard has its own conversion factor
Always verify the specific conversion factor for the compound in question, as using wrong conversions creates significant errors.
Concentration vs. Amount
Research may express measurements as concentrations (amount per volume) or total amounts:
Concentration Units
- mg/mL — milligrams per milliliter of solution
- mcg/mL — micrograms per milliliter
- IU/mL — international units per milliliter
Total Amount
- mg — total milligrams administered
- mcg — total micrograms
- IU — total international units
Concentration multiplied by volume gives total amount. Pay attention to which is being reported in research.
Per-Kilogram Expressions
Research often expresses amounts relative to body weight:
- mg/kg — milligrams per kilogram of body weight
- mcg/kg — micrograms per kilogram
This is common in animal studies and some human research. It accounts for the fact that larger subjects may respond differently than smaller ones, though the relationship is not always linear.
Common Errors to Avoid
- Confusing mcg with mg (1000x difference)
- Applying wrong IU conversion factors
- Confusing concentration with total amount
- Directly scaling animal per-kg doses to humans
- Assuming all sources use consistent standardization