Syringes, units and concentration, explained

Units are volume, not drug

An insulin syringe is marked in units because it was designed for insulin, where the concentration is standardized so that a unit of volume always contains the same amount of insulin. That convenience does not carry over to anything else drawn up in the same syringe.

For every other injectable, the mark you draw to describes an amount of liquid. What that liquid contains depends entirely on the concentration of the vial it came from. This is the single most important sentence on this page.

It follows that a number is meaningless on its own. Someone saying they draw to a particular mark tells you nothing useful unless you also know their vial, how it was prepared and what was prescribed, and even then it is their prescription rather than yours.

What concentration means

Many peptides are supplied as a freeze-dried powder in a sealed vial. Before use, a sterile liquid is added, the powder dissolves, and the result is a solution with a concentration: an amount of drug per milliliter of liquid.

The amount of drug in the vial is fixed by the pharmacy. The amount of liquid added is a choice. Add less liquid and the solution is more concentrated, so each mark on the syringe carries more drug. Add more and each mark carries less.

This is why two people can hold vials labeled identically, follow the same number on the same syringe, and receive different amounts. Nothing was faked and nobody miscounted. The concentrations were different.

• Powder in a vial has no concentration until liquid is added.
• Concentration = amount of drug in the vial divided by the volume of liquid added.
• Syringe marks measure volume, so the drug delivered depends on concentration.
• Different syringe sizes carry different scales, so the same printed number is a different volume.

Terms you will see on your instructions

None of these are complicated once separated out. Most of the confusion comes from the words being used loosely in videos and forums rather than from the ideas being hard.

• Milligram and microgram: masses of drug. A milligram is a thousand micrograms, which is why a misplaced decimal matters so much here.
• Milliliter: a volume of liquid. A standard small vial holds a few milliliters at most.
• Reconstitution: adding sterile liquid to a freeze-dried powder to make a solution.
• Bacteriostatic water: sterile water containing a preservative, used so a vial can be entered more than once. Sterile water without preservative is a different product with different handling.
• Insulin syringe: a fine, short needle with a volume scale marked in units, commonly used for small subcutaneous volumes.

Practical handling that is not dose math

There is plenty that is safe and useful to say. Storage, handling and technique cause a lot of real world problems and none of it involves calculating an amount.

Ask about anything on this list that your instructions do not already cover, and ask before your first injection rather than during it.

• Storage: most reconstituted peptides are refrigerated, and stability after mixing is limited. Follow the date your pharmacy or provider gives you rather than a general rule from the internet.
• Light and heat: keep vials in their box, and do not leave them in a car or a bathroom.
• Never share a vial, a syringe or a needle, and never reuse a needle. Needles blunt on first use, which makes the second injection worse.
• Rotate injection sites so the same patch of tissue is not used repeatedly.
• Dispose of sharps in a proper sharps container, not household waste.
• Inspect the solution before use. Anything cloudy, discolored or containing particles should be queried, not injected.
• If something looks wrong, or the instructions and the syringe do not obviously agree, do not proceed. Message us.

How this works on a Baseline program

A consult comes before an order, and an independent licensed provider in our network writes the prescription after reviewing your labs, your history and your goals. Baseline is a wellness consultancy and does not prescribe.

When a therapy is live, it is dispensed with written instructions specific to your vial and includes messaging access, which is where questions like this one are meant to land. There is no charge for asking and no such thing as a question too basic.

For patients who want the arithmetic laid out for their own vial, that will live behind your patient login alongside your prescription, and it is on our list to build.

References

1. Institute for Safe Medication Practices, guidance on look-alike volumes, unit-based measurement errors and tenfold dosing errors with subcutaneous injections
2. US Food and Drug Administration, safe disposal of sharps and single-use needle and syringe guidance
3. Baseline clinical policy: dose and reconstitution instructions are issued by the prescribing provider for the specific vial dispensed