Milligrams, Micrograms, and Units
Four different scales, measuring four different things, all abbreviated to short words that look interchangeable and are not. This is the shortest module in the track and the one with the highest cost of failure.
The four scales
| Scale | Measures | Where you meet it |
|---|---|---|
| Milligram (mg) | Mass | Vial labels, most commonly |
| Microgram (mcg / µg) | Mass, one thousandth of a mg | Doses, most commonly |
| Millilitre (mL) | Volume | Diluent, syringe capacity |
| Unit (U) | Depends entirely on context | Syringe barrel; some hormone labels |
The first three are unambiguous. The fourth is the problem, and it is the problem in two independent ways.
Mass: mg and mcg
The only relationship you need:
- 1 mg = 1,000 mcg
- Therefore 0.25 mg = 250 mcg, 0.1 mg = 100 mcg, 2.5 mg = 2,500 mcg.
- 1 g = 1,000 mg
- Rarely relevant here, but completes the ladder.
The failure mode is the factor of a thousand. A vial labelled 5 mg contains 5,000 mcg. Someone reading a protocol written in mcg while holding a vial labelled in mg, and not converting, is off by three orders of magnitude. This is not a hypothetical error; it is the most frequently reported one in this field, and it goes in both directions: people take a thousandth of what they meant to and conclude the compound does nothing, or a thousand times and conclude something much worse.
Adopt a habit: convert everything to micrograms before doing any arithmetic. Milligrams are for labels. Micrograms are for thinking.
Volume: mL and cc
A millilitre and a cubic centimetre are the same volume. Older syringes and older writing use "cc"; they are interchangeable and nothing turns on which you see. Volume is the easy scale. It is also the only thing a syringe can actually measure, which brings us to the fourth item.
The unit, problem one: the syringe barrel
This is the most important paragraph in the module, so it gets its own line.
A "unit" on an insulin syringe is a measure of volume, not a measure of drug.
An insulin syringe marked U-100 is calibrated for insulin formulated at 100 international units per millilitre. The manufacturer divided one millilitre into a hundred equal marks so that a diabetic patient could draw "twelve units" without doing arithmetic. The mark is convenient shorthand for a volume:
- 1 unit on a U-100 barrel = 0.01 mL
- Because 100 marks span 1 mL.
- 10 units = 0.1 mL · 50 units = 0.5 mL · 100 units = 1.0 mL
- Nothing about this depends on what is in the syringe.
The mark tells you how much liquid you have drawn. How much compound is in that liquid depends entirely on the concentration you created when you reconstituted the vial, which is Module 05, and which is why that module cannot be skipped.
The specific error this produces: someone reads "dose 250 mcg", draws to the 250 mark, except there is no 250 mark, so they draw to 25 on a barrel that only goes to 100, and they have now injected a quarter of a millilitre of something whose concentration they never calculated. The number on the barrel and the number in the protocol are measuring different physical quantities. They are never the same number by anything but coincidence.
U-40 syringes exist
Veterinary insulin is often formulated at 40 IU/mL, and U-40 syringes are calibrated to match: 40 marks span 1 mL, so each mark is 0.025 mL. A U-40 syringe looks very much like a U-100 syringe. If you reason with U-100 arithmetic while holding a U-40 barrel, every volume you draw is two and a half times what you intended.
Check the barrel. The calibration is printed on it. This takes two seconds and eliminates an entire category of error.
Barrel sizes and how finely they read
| Barrel | Capacity | Typical graduation | Smallest readable volume |
|---|---|---|---|
| 0.3 mL (30 unit) | 30 units | Every 1 unit, often half-units | ~0.005-0.01 mL |
| 0.5 mL (50 unit) | 50 units | Every 1 unit | ~0.01 mL |
| 1.0 mL (100 unit) | 100 units | Every 2 units | ~0.02 mL |
The practical implication is that a small barrel is more accurate for small volumes, and that any dose landing between two marks on a 1 mL barrel is being estimated rather than measured. Module 05 shows how to choose a diluent volume deliberately so that your doses land on marks you can actually read.
The unit, problem two: the International Unit
An International Unit (IU) is an entirely different animal from a mark on a barrel. It is a measure of biological activity, defined separately for each substance by international agreement. One IU of insulin and one IU of growth hormone have no relationship to each other whatsoever. They are not the same mass, not the same potency, not the same anything. The unit is defined per substance.
This means there is no general conversion between mg and IU. Anyone who offers you one as a universal rule has misunderstood the concept. Per-substance conversions do exist and are published for the substances that use IU at all. Somatropin, for example, is conventionally taken at approximately 3 IU per mg, so a 10 mg vial is described as roughly 30 IU. That figure applies to somatropin and to nothing else.
Most research peptides are not standardised in IU at all and are labelled purely by mass. If you see IU on a peptide label, find out what standard is being invoked before you convert anything.
"Unit" can mean a mark on a barrel (a volume), an International Unit (a quantity of biological activity), or, in casual writing, simply "the amount I take". When you read the word, determine which one is meant before you do anything with it. When you write it, say which one you mean.
Concentration is the bridge
Mass and volume are connected by exactly one quantity: concentration, expressed in mg/mL or mcg/mL.
- Concentration = Mass ÷ Volume
- 5 mg in 2 mL is 2.5 mg/mL.
- Mass = Concentration × Volume
- 2.5 mg/mL drawn to 0.1 mL delivers 0.25 mg, i.e. 250 mcg.
- Volume = Mass ÷ Concentration
- To deliver 250 mcg from a 2,500 mcg/mL solution, draw 0.1 mL, which is 10 units.
Every dosing calculation you will ever do is one of those three lines. Module 05 does nothing but apply them.
Reference conversions
| Units on U-100 barrel | Volume | At 1 mg/mL | At 2.5 mg/mL | At 5 mg/mL |
|---|---|---|---|---|
| 2 | 0.02 mL | 20 mcg | 50 mcg | 100 mcg |
| 5 | 0.05 mL | 50 mcg | 125 mcg | 250 mcg |
| 10 | 0.10 mL | 100 mcg | 250 mcg | 500 mcg |
| 20 | 0.20 mL | 200 mcg | 500 mcg | 1,000 mcg |
| 25 | 0.25 mL | 250 mcg | 625 mcg | 1,250 mcg |
| 50 | 0.50 mL | 500 mcg | 1,250 mcg | 2,500 mcg |
What you should be able to do now
- Convert between mg and mcg without hesitating.
- State what a mark on a U-100 barrel physically measures, and identify a U-40 barrel on sight.
- Explain why there is no universal mg-to-IU conversion.
- Write down the three forms of the concentration relationship from memory.
Module 05 puts all of it together on a real vial.