29G, 30G or 31G: which should I buy?

General information about the equipment, not medical advice. The directions that came with your medicine and the person who prescribed it come first, every time.

What actually changes between them

Very little, and that is worth saying plainly. All three are fine needles in the same family, and the difference between them is a fraction of a millimetre of outer diameter. Nobody is going to get a different result from their medicine because they chose 30G over 31G.

What does change is comfort and draw time. A 31G needle is the most comfortable of the three and the slowest to pull liquid through. A 29G fills noticeably faster and you will feel it more. Whether that trade is worth it depends on how much you are drawing and how often.

When to go thicker on purpose

Viscosity is the main reason. A thin watery solution moves through a 31G without complaint. An oil moves very slowly, sometimes slowly enough that you give up and squeeze, which is how a plunger gets forced and a dose gets lost.

The usual answer for anything viscous is to draw up with a wider needle and swap to a finer one to inject. A drawing-up needle is blunt and wide, which fills a barrel in seconds rather than a minute, and it cannot be injected with by accident. It is a second needle per dose and it is worth it.

The other reason is dexterity. Very fine needles bend if they meet resistance and are fiddly to handle. If you find that frustrating, a 29G is not a worse choice, it is a different one.

Length matters more than gauge

If you are trying to decide between these three, you are optimising the thing that matters least. Length decides which layer of tissue the tip reaches, and that is a genuine clinical question rather than a comfort preference.

Australian and international guidance has moved towards shorter needles for injections under the skin over the last decade, which is why 4mm pen needles are now standard. Which length suits you is a question for your prescriber.