The quick answer

How you are injecting Syringe Needle
Subcutaneous, when prescribed and taught Insulin syringe: the smallest barrel that holds your volume Fixed 31G, 6mm may suit small volumes
Intramuscular, usually clinician-taught 1mL or 3mL Luer-lock syringe Often 23 to 25G and 25 to 38mm, depending on product, site, depth and build

If your prescriber has told you to inject subcutaneously at home and confirmed the product suits it, a 31G, 6mm insulin syringe may suit small volumes. The needle is moulded into the barrel, so one syringe can draw from a single vial and inject, without a transfer step. Don't use fixed-needle insulin syringes where you need to draw from an ampoule or several vials, or use a filter or transfer needle.

Subcutaneous TRT supplies laid out: a 31G 6mm insulin syringe, individually wrapped alcohol wipes, and a locking 1.8L sharps container
Subcutaneous TRT supplies laid out: a 31G 6mm insulin syringe, individually wrapped alcohol wipes, and a locking 1.8L sharps container.

Why needle size matters for TRT

Gauge is the needle's diameter: the higher the number, the thinner the needle. Testosterone esters come dissolved in oil, which is thicker than water-based medicines, so a fine 31G needle draws slowly. That's expected. Don't change gauge or route unless your prescriber or pharmacist recommends it.

Length matters because a subcutaneous dose should land in fat, not muscle and not the skin. A 6mm needle suits many small subcutaneous doses, though your build, site and technique still count. Oil-based testosterone forms a depot where it's injected, and absorption varies with route and depth, so use the route you were prescribed.

Which barrel: 0.5mL or 1mL?

Both are 31G, 6mm. Choose by how much you inject each time, since a smaller barrel has finer marks.

Volume (mL) = prescribed mg per injection divided by concentration (mg/mL). On a U-100 syringe, mL x 100 = unit marks. Have your prescriber or pharmacist confirm the result.

  • 0.5mL insulin syringe: for volumes up to 0.5mL (50 unit marks), with finer marks for accurate small doses.
  • 1mL insulin syringe: for volumes above 0.5mL, or when your prescriber or pharmacist recommends it.
A 0.5mL and a 1mL insulin syringe side by side, showing the finer dose graduations on the smaller 0.5mL barrel
A 0.5mL and a 1mL insulin syringe side by side, showing the finer dose graduations on the smaller 0.5mL barrel.

Our insulin syringe sizes guide explains how to read the barrel and match it to your dose.

What about intramuscular TRT?

Intramuscular (IM) injections go deeper, into muscle, often with a 23 to 25G, 25 to 38mm needle. Your prescriber or pharmacist chooses the exact size for your product, site, depth and build. A clinician or nurse normally teaches IM technique in person.

We don't currently stock IM needles. If you're on an IM protocol, ask your pharmacy about drawing and injecting needles and technique.

Supplies for subcutaneous TRT

Browse the full injection supplies collection. These supplies don't need a prescription in Australia. Testosterone does, and must be used under clinician direction.

When to call your prescriber

A drop of blood or a brief sting is normal. Stop and contact your prescriber for:

  • Redness, warmth, swelling or pus at the site over the next 24 to 72 hours, which may be infection
  • A hard lump that doesn't settle after a week, or one that is hot and tender
  • Fever, chills or feeling unwell after an injection
  • A rash, hives or itching at or beyond the site (possible allergic reaction)
  • Drawing from a vial that was cloudy, had particles or was stored wrongly

Signs of a severe allergic reaction - swelling of the face or throat, or trouble breathing - call 000 immediately.

Frequently asked questions

What size needle is best for subcutaneous testosterone?

If your prescriber has approved subcutaneous TRT for your product, a 31G, 6mm insulin syringe may suit small volumes.

Can I use an insulin syringe for testosterone?

Yes, when your prescriber has set you up for subcutaneous injection. The barrel is marked in U-100 units (100 marks = 1mL).

Why does the oil draw so slowly?

Oil is thick and fine needles draw it slowly. If your pharmacist says it's fine, warm the vial briefly in clean hands. Never use hot water or a microwave, and return it to its labelled storage afterwards.

Do I need a separate drawing needle for SubQ TRT?

Not usually, if you draw from a single vial with a fixed-needle insulin syringe. Some prescribed setups use detachable draw needles. Never draw from more than one vial with the same needle or syringe. Follow your pharmacist for multi-vial, ampoule, filter or transfer setups. Repeated punctures with large needles can damage the stopper.

This guide helps you choose injection supplies and is not medical advice. Follow your prescriber's dose, concentration and route, and keep up your regular TRT monitoring.