Choosing needles and syringes

A syringe wrapper carries three numbers and none of them are labelled. A packet marked 31G 8mm 0.5mL is telling you how thick the needle is, how long it is, and how much the barrel holds, in that order. Getting the third number wrong is the one that causes dosing errors, because a barrel too large for the dose puts the measurement down at the bottom of the scale where nobody can read it accurately.

This guide explains what the numbers mean and how people choose between them. It describes equipment rather than recommending that anyone use it.

Gauge: the counter-intuitive number#

Gauge measures the outside diameter of the needle, and the scale runs backwards. A higher gauge number is a thinner needle. A 31G needle is finer than a 27G one, which is finer than an 18G one.

Thinner is more comfortable and does less tissue damage, and it is also slower, because the flow through a tube falls off sharply as the bore narrows. That trade is the reason two different needles usually get used for two different jobs.

• Drawing liquid from a vial favours a wider bore. Something in the 18G to 21G range moves bacteriostatic water in seconds rather than minutes, and it passes through a rubber stopper cleanly.
• Injecting subcutaneously favours a fine bore. Insulin syringes are typically 29G to 31G, and the volume involved is small enough that the slower flow costs nothing.

An insulin syringe has its needle bonded to the barrel and cannot be swapped, so where both jobs use one syringe, they are done with the same fine needle. That works, and the cost is a slow draw and a needle that is slightly blunted by the stopper before it reaches skin.

Length, and what sits underneath the skin#

Needle length decides which tissue the liquid lands in. Subcutaneous injection targets the fat layer between the skin and the muscle, and a needle long enough to pass through that layer delivers into muscle instead, which is a different procedure with a different absorption profile.

Common lengths for subcutaneous use are 4mm, 6mm and 8mm. Diabetes practice has moved steadily toward the shorter end over the past two decades, on evidence that 4mm and 6mm needles reach the subcutaneous layer reliably in adults across a wide range of body types while reducing accidental intramuscular delivery. Longer needles, 12.7mm and above, are intramuscular equipment.

Length interacts with technique. A short needle is generally used at 90 degrees without pinching; a longer one is where pinching the skin and angling the needle starts to matter. That is covered in the injection technique guide.

Barrel size, and the unit markings#

Insulin syringes are marked in units rather than millilitres, on the U-100 standard: 100 units to 1 mL. That means one unit is 0.01 mL, and the conversion never changes regardless of what is in the barrel.

The unit is a volume measurement here, not a dose. It says how much liquid is in the syringe and nothing about how much peptide that liquid contains, which depends entirely on how the vial was mixed. Two people drawing "10 units" of the same compound can be taking very different amounts.

Three barrel sizes are common:

• 0.3 mL, marked to 30 units. The finest graduations, often with half-unit marks. The best choice for small doses, because the same dose spans more of the scale.
• 0.5 mL, marked to 50 units. A middle option, one unit per mark.
• 1 mL, marked to 100 units. Needed for larger volumes, and the worst of the three for reading a small dose accurately.

The rule that follows from this is to use the smallest barrel that will hold the dose. A 6-unit dose measured in a 1 mL syringe sits in the bottom 6% of the barrel, where the plunger seal is wide, the markings are coarse and parallax error is at its worst. The same dose in a 0.3 mL syringe is a fifth of the way up a finer scale.

The reconstitution calculator shows the fill level for each of the three barrel sizes, which makes the choice visible rather than theoretical.

Coring, and why the same spot twice is a bad idea#

Pushing a needle through a rubber stopper can punch out a small disc of the rubber and carry it into the vial. This is called coring, and a cored fragment floating in a solution is both a contamination problem and a sign the stopper is losing its seal.

Two things reduce it. Entering at a slight angle rather than straight down lets the needle bevel cut rather than punch, and varying the entry point spreads the punctures across the stopper instead of working a hole through one spot. A stopper that has been entered many times in the same place is worth retiring the vial over.

Single use means single use#

A needle is sterile until it is opened and sharp until it is used once. Both change on the first pass.

Under a microscope, a used needle tip is visibly hooked and burred, and a second insertion tears rather than cuts. That is the mechanism behind a good deal of the bruising, scarring and tissue thickening that shows up at injection sites. A needle that has been through a rubber stopper has already been blunted before it reaches skin, which is a further argument for drawing up with one needle and injecting with another where the equipment allows it.

Sharps disposal#

Used needles go into a rigid, puncture-resistant sharps container. They do not go into household bins, recycling, or a drinks bottle, and the reason is that somebody handles that waste further down the line.

Sharps containers are sold by pharmacies and online. What happens when one is full varies by country and often by local authority: some areas run a collection service, some pharmacies take them back, and some require a specific disposal route. It is worth finding out which applies where you live before the container is full rather than after.

Recapping a used needle is where a large share of accidental needlestick injuries happen, which is why clinical practice is to drop it straight into the container without recapping.

Frequently asked questions#

What does the dead space on a syringe mean?

Dead space is the liquid left in the needle and hub after the plunger is fully depressed, and it is not delivered. Insulin syringes with bonded needles have very little of it, which is one of the reasons they are used for small doses.

Is a thinner needle always better?

For comfort, generally yes. For drawing liquid out of a vial, no: a 31G needle will move bacteriostatic water, but slowly enough that people are tempted to force the plunger, and forcing liquid through a fine needle into a vial is exactly the jet that damages a peptide during reconstitution.

Do syringes expire?

They carry an expiry date, and it refers to the sterility of the packaging rather than the plastic. A sealed syringe past its date has an unverified sterile barrier, and an opened one has none at all.