If you've used different injection pens, you may have noticed that some medications are dosed in milligrams (mg) while others use International Units (IU or simply "Units"). These are not interchangeable units you can convert with a simple formula. They represent two fundamentally different ways of quantifying a drug, and understanding this distinction is crucial for safety, efficacy, and product design.
Part 1: The Fundamental Difference – "Mass" vs. "Biological Activity"
This is the core principle:
Milligrams (mg): The Measure of Mass
Examples: Most modern biologic drugs like GLP-1 agonists (e.g., semaglutide, liraglutide) are typically dosed in mg (e.g., 0.25 mg, 0.5 mg, 1.0 mg).
What it is: A milligram is a universal metric unit of mass (1/1000th of a gram).
What it measures: It measures the precise physical-chemical weight of the active drug substance. It tells you how many grams of the actual protein or molecule are present.
Used for: Drugs with a well-defined, stable molecular structure that can be precisely quantified by chemical analysis.
International Units (IU): The Measure of Biological Effect
Classic Examples:
Insulin: This is the most common example. 1 IU of insulin is defined by its ability to lower blood glucose in a standardized rabbit assay. Different insulin analogs (aspart, glargine, detemir) have different molecular weights, but are calibrated so that 1 IU produces the same predictable glucose-lowering effect.
Growth Hormone, Heparin, certain interferons, and vitamins (like Vitamin D) are also measured in IU.
What it is: An IU is a unit of measurement for the biological effect or "potency" of a substance, based on an internationally agreed-upon biological assay (test).
What it measures: It measures the drug's ability to produce a specific, standardized biological response in a test system, NOT its weight. The mass equivalent of 1 IU is completely different for each substance.
Used for: Complex substances whose potency is best defined by their action in a biological system, often due to historical reasons or molecular complexity.
Part 2: Why Do Injection Pens Use Different Units?
The choice stems from the nature of the drug and clinical necessity:
Predictable Clinical Outcome: For drugs like insulin, the primary goal is a reliable, predictable effect on blood sugar. Using IU ensures that a dose of "10 units" delivers the same biological activity and clinical result, regardless of the minor variations in the actual molecular weight between batches or manufacturers. Using mg could lead to unpredictable efficacy.
Historical Standardization: Many biologics (like insulin) were discovered and used therapeutically long before their exact molecular structure was known. The IU system created a global, stable standard for dosing based on effect, ensuring continuity of care for decades.
Modern Precision: For newer, synthetically produced biologics like semaglutide, the molecule is perfectly defined, pure, and its mass has a consistent relationship to its effect. Therefore, using mg is more straightforward, scientific, and intuitive for prescribers.
Part 3: The Relationship & The Critical "Conversion" Misconception
There is NO universal conversion factor. You cannot convert mg to IU like you convert kilograms to pounds. The relationship is substance-specific.
The relationship is "fixed" and "defined": For a specific drug, there is a fixed conversion factor established by the manufacturer through rigorous biological testing and approved by regulatory authorities (FDA, EMA, etc.).
For a specific insulin, the conversion might be: 1 mg = 28 IU (Example only. Refer to the specific product label).
For recombinant human growth hormone, the standard is approximately: 1 mg = 3 IU.
This factor is a core property of the product and is stated in its regulatory filing and prescribing information.
Example:
How it Works in an Injection Pen: The pen's dose selector mechanism and display window are engineered based on this fixed, internal conversion.
A pre-filled pen containing 3 mg of a drug with a conversion of 1 mg = 30 IU would contain a total of 90 IU of biological activity.
When you dial the pen to "30" on the IU scale, you are instructing it to deliver 1 mg of the drug. The pen's internal mechanism is designed to move the plunger the exact distance needed to expel that corresponding mass.
Copyright © Shijiazhuang Xinfuda Medical Packaging Co., Ltd. All Rights
MAKE AN ENQUIRY
