Pen Injector Users' Guide: mg vs. IU

Is it 30 units or 1 mg? A simple guide to decoding your pen's dosage display.

Have you ever picked up a new injection pen and wondered, "Why does this pen say mg, but my old one says units or IU?" You're not alone. This is one of the most common questions we hear from patients starting new therapies.

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Understanding the difference isn't just medical trivia-it's crucial for using your pen safely and effectively. Let's decode these two measurement systems in simple terms.

The Core Difference: "Weight" vs. "Effect"

Think of it this way: mg tells you how much "stuff" is in the shot, while IU/units tell you how powerful that "stuff" is.

Milligrams (mg): The Scale Measurement

What it means: Directly measures the physical weight of the medication in the pen.

Analogy: Like weighing flour for a recipe in grams.

What you need to know: If your pen says mg, you're looking at the actual weight of the drug molecules in your dose.

Common uses: Newer biologic drugs (e.g., GLP-1 medications like Ozempic, Wegovy, Mounjaro).

International Units (IU) or Units (U): The Power Measurement

What it means: Measures the biological activity or "potency" of the medication.

Analogy: Like measuring the brightness of light bulbs in "lumens"—different bulb types give different brightness even at the same wattage.

What you need to know: If your pen says units, you're looking at the drug's proven ability to produce a specific effect in your body.

Common uses: Insulin, growth hormone, some older biologic drugs.

Visual Comparison: mg vs. IU Pens

Why This Matters for You: The Safety Implications

1. NEVER Convert Between Them Yourself

This is the most important rule: You cannot convert mg to units with a simple formula. The conversion is different for every single medication.

Example: 1 mg of insulin ≠ 1 mg of semaglutide ≠ 1 mg of growth hormone

What this means for you: If your doctor prescribes "1 mg" of a medication, dial exactly "1" on a mg pen. If they prescribe "10 units," dial exactly "10" on a units pen. Never do the math yourself.

2. Pens Are NOT Interchangeable

A unit-based insulin pen and an mg-based GLP-1 pen are as different as a car and a motorcycle-even if they look similar.

True story: A patient once tried to use their insulin pen (set to 30 units) to inject a GLP-1 medication that was prescribed as 0.5 mg. This could have caused severe side effects because 30 units of insulin is a completely different dose than 0.5 mg of a GLP-1 drug.

3. Consistency Is Everything

Units pens ensure that every "unit" gives you the same biological effect, even if the drug molecules vary slightly.

mg pens ensure you get exactly the same physical amount of drug molecules each time.

Both approaches are designed to keep your treatment consistent and effective.

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Quick Guide: Recognizing Your Pen Type

If Your Pen Shows:

Decimals (0.25, 0.5, 1.0, 2.0) → Likely mg

Whole numbers only (5, 10, 15, 30, 60) → Likely units

Small numbers (0.25 to 2) → Probably mg

Larger numbers (5 to 100+) → Probably units

Common Examples:

Your pen's measurement system (mg or units) is not your math problem to solve. It's the manufacturer's job to ensure that when you dial "1" on your pen, you get exactly what your doctor prescribed.

The beauty of modern injection pens is that they handle the complex science for you. Your job is simply to:

Use the right pen for your prescribed medication

Dial the exact number your doctor prescribed

Inject confidently, knowing the pen delivers precise medicine