Guide

How to read your prescription

What the abbreviations mean, and the mg-versus-mL distinction that is the one genuine hazard in this category.

Illustration of an open book with a path drawn across it

Updated 2026-09-18 · every price checked by hand

Reading your own paperwork is free, and it catches the two mistakes that cost the most: paying for a formulation you did not want, and a quantity that does not match what you were charged for.

That part is the difference between your dose in milligrams and the volume you draw in millilitres. They are not the same number, the relationship between them depends on the concentration of your particular vial, and compounded preparations vary in concentration between pharmacies and sometimes between refills. Getting this wrong is the one way to seriously harm yourself with a correctly-dispensed medication.

Checking you got what you paid for

Match the quantity dispensed against the days supply and against your invoice. A 28-day supply that looks like a fortnight is worth querying before you inject, not after. And check the formulation — oral, sublingual and injectable are priced very differently, and the cheapest advertised tier is frequently not the one people think they bought.

The anatomy of a prescription

What it says	What it means
Drug name and strength	The medication and its concentration, e.g. "tirzepatide 10 mg/mL". The second half is the bit that matters.
Sig	Latin for "label" — the directions for you. This is the line telling you how much and how often.
Disp / Quantity	How much is being dispensed, usually in mL for an injectable.
Refills	How many times it can be refilled before you need a new prescription.
Days supply	How long the quantity should last. Useful for spotting an error — a 28-day supply that looks like two weeks is worth querying.
NDC	National Drug Code, identifying the exact product. Compounded preparations may not carry one.
DAW	"Dispense as written" — the prescriber does not want it substituted.

The mg-versus-mL problem, worked through

Your dose is prescribed in milligrams. Your syringe measures millilitres, or units. Converting between them requires the concentration printed on the vial.

An illustration, not your instructions. If a vial reads 10 mg/mL, then 1 mL contains 10 mg. A 5 mg dose would be 0.5 mL. If the pharmacy sends a refill at 20 mg/mL instead, that same 0.5 mL now contains 10 mg — double the dose — even though nothing about your prescription changed.

Check the concentration on every refill. Compare the mg/mL on a new vial against your last one before you draw anything. If it has changed and nobody told you, stop and contact the provider. Never work out your own volume from a previous vial — confirm it against the concentration in front of you, and ask the pharmacy if there is any doubt at all.

What is different about a compounded prescription

It is prepared for you individually, so it should carry your name and a patient-specific direction.
It carries a beyond-use date rather than a manufacturer expiry, and that date is typically much shorter — check it against how long your supply needs to last.
Concentration is set by the compounding pharmacy, not a manufacturer, so it genuinely can differ between pharmacies.
It may not carry an NDC.
It should identify the compounding pharmacy. If it does not, ask who made it.

Red flags on a prescription

No concentration stated anywhere. You cannot dose safely without it.
No prescriber name, or no way to contact them.
No pharmacy identified on a compounded preparation.
Directions in "units" with no explanation of what a unit means on your syringe.
A quantity that does not match the days supply.
Anything sold to you without a prescription at all — see how to spot a bad seller.

Common questions

My syringe is marked in units, not mL. What now?

Insulin syringes are marked in units where 100 units = 1 mL. That conversion plus your concentration determines the volume. This is exactly the calculation to confirm with the pharmacy rather than work out alone — they will do it with you over the phone, at no charge.

Can I ask a pharmacist even if I did not buy it there?

Yes. Pharmacists answer this kind of question for free and it is a good use of them. Bring the vial and the paperwork.

The concentration changed and my provider says it is fine. Is it?

It can be entirely legitimate — but the volume you draw must change with it. Get the new volume in writing, and make sure you understand it before the next injection rather than after.

General information, not medical advice. Your prescriber and dispensing pharmacist are the authority on your own prescription, and no website is a substitute for asking them.