Peptide labels and prescriptions use three small abbreviations that cause a lot of confusion: mg, mcg and IU. On top of that, the syringe in your hand is marked in "units," which is a fourth idea. This guide explains each one in plain English so you can read your label and talk to your pharmacist with confidence.
Key takeaways
- mg (milligram) and mcg (microgram) are units of weight. 1 mg = 1,000 mcg.
- IU (international unit) measures biological activity. There is no single IU-to-mg conversion; it depends on the substance.
- The "units" printed on a U-100 insulin syringe measure volume: 100 units = 1 mL.
- Most mistakes happen when people mix these up, so write units out in full and double-check with your pharmacist.
Milligrams (mg): the everyday unit
A milligram is one-thousandth of a gram. Many prescription peptides are labeled in milligrams. For example, the FDA-approved GLP-1 pens for semaglutide and tirzepatide list their strengths in mg on the label.
When a vial says "5 mg," it means the vial holds 5 milligrams of the active ingredient in total. That total gets spread out once liquid is added, which is why concentration matters (more on that below).
Micrograms (mcg): a thousand times smaller
A microgram is one-thousandth of a milligram. Some peptides are measured in such tiny amounts that writing them in mg would mean lots of decimals, so micrograms are used instead.
The conversion is simple:
- mg to mcg: multiply by 1,000
- mcg to mg: divide by 1,000
| In mg | In mcg |
|---|---|
| 0.1 mg | 100 mcg |
| 0.25 mg | 250 mcg |
| 0.5 mg | 500 mcg |
| 1 mg | 1,000 mcg |
| 2.5 mg | 2,500 mcg |
These rows are just unit conversions. They are not suggested doses for anything.
Why "mcg" and not "µg"?
You may see micrograms written as "µg." Safety groups such as the Institute for Safe Medication Practices (ISMP) advise writing "mcg" instead, because a handwritten "µg" can be misread as "mg." That mistake would make a dose 1,000 times larger. If your paperwork uses µg, ask your pharmacist to confirm.
International units (IU): measuring activity, not weight
An international unit measures how much biological effect a substance has. It is used for things whose strength is easier to describe by activity than by weight, such as some hormones (like hCG), insulin and certain vitamins.
The key point: there is no universal IU-to-mg formula. Each substance has its own conversion, and some have none that is useful in daily life.
Vitamin D shows how it works. The NIH Office of Dietary Supplements lists adult intake as "15 mcg (600 IU)," which means that for vitamin D, 1 mcg equals 40 IU. That ratio only applies to vitamin D. Using it for any other substance would give a wrong answer.
ISMP also lists "IU" as an error-prone abbreviation, because it can be misread as "IV" or the number 10. Many pharmacies now write "units" instead.
Syringe "units" are a different thing
This is where most confusion starts. A U-100 insulin syringe is marked in units, but those marks measure volume:
- 100 units on a U-100 syringe = 1 mL
- 10 units = 0.1 mL
- 1 unit = 0.01 mL
For U-100 insulin, which is made at 100 units of insulin per mL, the syringe marks and the dose line up perfectly. For anything else, the syringe marks are just a ruler for volume. A peptide prescribed in mg or mcg has to be converted to mL first, and then to syringe units. Our guide on how to read a U-100 insulin syringe covers the markings in more detail.
Putting it together: from label to syringe
If your prescriber gives you a dose in mg or mcg and you have a vial you mixed yourself, there are three steps. The example below is arithmetic only. Your prescriber sets the actual dose.
Example (numbers chosen only to show the math): A vial holds 5 mg. It is mixed with 2 mL of bacteriostatic water. Your prescriber's instructions say a dose of 250 mcg.
- Find the concentration. 5 mg ÷ 2 mL = 2.5 mg per mL.
- Match the units. 250 mcg ÷ 1,000 = 0.25 mg.
- Find the volume. 0.25 mg ÷ 2.5 mg/mL = 0.1 mL.
- Convert to U-100 syringe units. 0.1 mL × 100 = 10 units.
Check it backward: 10 units is 0.1 mL, and 0.1 mL × 2.5 mg/mL = 0.25 mg, which is 250 mcg. The numbers agree.
Step 2 is the one people skip. If you divide 250 by 2.5 without converting mcg to mg first, you get 100, which is a very different (and wrong) result. Our reconstitution calculator does these steps for you and shows the work, so you can compare its answer with your pharmacist's instructions.
Common mix-ups to watch for
- mg read as mcg (or the reverse). A 1,000-fold difference. Always say the unit out loud when you read a label.
- Missing leading zero. ".5 mg" can be misread as "5 mg." Safe practice is to write "0.5 mg."
- Trailing zero. "1.0 mg" can be misread as "10 mg." Safe practice is to write "1 mg."
- Treating syringe units as mg. "Draw 20 units" is a volume (0.2 mL on a U-100 syringe), not 20 mg or 20 mcg.
- Borrowing an IU conversion from another product. Every substance is different.
Tips for staying accurate
Write your dose in full words in your notes, like "250 micrograms," not just a number. Keep the vial strength and the amount of water you added in the same place. When something looks off, stop and call your pharmacist before injecting. You can also log the vial strength, mixing date and each dose in the PepSteady app, so the numbers are in one place at every refill.
The bottom line
Milligrams and micrograms are weights, and 1 mg is 1,000 mcg. International units measure activity and have no single conversion. Syringe units measure volume, with 100 units equal to 1 mL on a U-100 syringe. Keep these four ideas separate, convert one step at a time, and let your prescriber and pharmacist confirm the final number.
Sources
- ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations
- NIH Office of Dietary Supplements: Vitamin D Fact Sheet for Consumers
- MedlinePlus: Giving a subcutaneous injection
- DailyMed: Ozempic (semaglutide) prescribing information
Frequently asked questions
How many mcg are in 1 mg?
There are 1,000 micrograms (mcg) in 1 milligram (mg). To turn mg into mcg, multiply by 1,000. To turn mcg into mg, divide by 1,000.
Can I convert IU to mg with one formula?
No. An international unit (IU) measures biological activity, and the conversion is different for every substance. Only use a conversion your product label, prescriber or pharmacist gives you for that exact product.
Are the units on an insulin syringe the same as IU?
No. The numbers on a U-100 insulin syringe measure volume, where 100 units equals 1 mL. They only match a drug's dose in units when the drug itself is made at 100 units per mL, like U-100 insulin.