What Size Needle for Testosterone (TRT) Injections in Australia?
The quick answer
| How you are injecting | Syringe | Needle |
|---|---|---|
| Subcutaneous (into the fat layer - when prescribed and taught) | Insulin syringe: choose the smallest barrel that comfortably holds the prescribed mL volume | Integrated 31G, 6mm may suit some small-volume SubQ prescriptions |
| Intramuscular (into the muscle - usually clinician-taught) | 1mL or 3mL Luer-lock syringe | Often 23-25G and 25-38mm, but exact sizing depends on product, site, injection depth, and patient build |
If your prescriber has specifically instructed subcutaneous testosterone at home (subcutaneous = into the fat layer just below the skin) and confirmed the product is suitable, a 31G, 6mm insulin syringe may suit small prescribed volumes. The needle is moulded into the barrel, so one fine syringe can draw from a single suitable vial and inject without a separate transfer step when your prescriber or pharmacist has approved that workflow. Do not use fixed-needle insulin syringes where an ampoule, filter needle, transfer needle, or multiple-vial draw is required.
Why needle size matters for TRT
Gauge is the needle's diameter, and a higher number means a thinner needle. 31G is a fine option that some people use for small subcutaneous volumes, but it is not universal for oil-based testosterone. Testosterone esters are usually dissolved in an oil-based vehicle, which is thicker than water-based medications. Slow drawing is expected; do not change gauge or route unless your prescriber or pharmacist recommends it.
Length matters because a subcutaneous injection is intended for the fat layer, not the muscle beneath it and not too shallowly in the skin. A 6mm needle suits many small-volume subcutaneous prescriptions, but body habitus, site, and technique still matter. Oil-based testosterone forms a depot at the injection site; absorption can vary by route and depth, so use the route your prescriber specified.
Which barrel - 0.5mL or 1mL?
Both are 31G, 6mm. The choice is about how much you inject per shot, because a smaller barrel has finer graduations and measures small volumes more precisely.
Work out your per-injection volume with the formula your prescriber or pharmacist confirms: prescribed mg per injection divided by concentration in mg/mL = mL. For U-100 insulin syringes, mL multiplied by 100 = U-100 unit marks.
- 0.5mL insulin syringe - useful for small prescribed volumes up to 0.5mL (50 U-100 unit marks). The finer-graduated barrel can make small volumes easier to measure accurately.
- 1mL insulin syringe - useful when the prescribed volume is above 0.5mL or when your prescriber or pharmacist recommends the larger barrel. Choose the smallest barrel that comfortably holds the prescribed volume.
If you are not sure which fits your prescription, our insulin syringe sizes guide walks through reading the barrel and matching it to your dose.
What about intramuscular TRT?
Some prescriptions are intramuscular (IM) - a deeper injection into the muscle, often with a longer needle in the 23-25G and 25-38mm range. The exact gauge and length must be selected by your prescriber or pharmacist for the product, site, injection depth, and patient build. IM technique is normally taught in person by a clinician or nurse.
We do not currently stock IM injection needles. If your prescriber has you on an IM protocol, speak to your pharmacy about the right drawing and injecting needles and about technique. This guide and the supplies below are written for the subcutaneous route.
Supplies for subcutaneous TRT
- 1mL insulin syringes (31G, 6mm) - one fresh syringe can draw and inject for appropriate small-volume SubQ prescriptions.
- 0.5mL insulin syringes (31G, 6mm) - finer graduations for smaller per-injection volumes.
- Alcohol wipes (200-pack) - one for the vial stopper, one for the site, every time.
- 1.8L Sharpsafe container - a locking sharps bin for home injection waste; replace and dispose of it according to the fill line and local sharps disposal guidance.
You can also browse the full injection supplies collection. The listed supplies are generally available without a prescription in Australia; testosterone medicines require a valid prescription and must be used under clinician direction.
When to call your prescriber
Most subcutaneous injections go smoothly - a small drop of blood or a brief sting is normal. Stop and contact your prescriber if you notice any of these:
- Redness, warmth, swelling or pus at the injection site that develops over the next 24 to 72 hours (possible infection).
- A hard lump that does not settle after a week, or one that is hot and tender.
- Fever, chills or feeling generally unwell after an injection.
- A rash, hives or itching at or beyond the site (possible allergic reaction).
- Signs of a severe allergic reaction - swelling of the face or throat, or trouble breathing - call 000 immediately.
- You realise you drew from a vial that was cloudy, contained visible particles, or had been stored wrongly.
Frequently asked questions
What size needle is best for subcutaneous testosterone?
If your prescriber has specifically instructed subcutaneous TRT and confirmed the product is suitable, a 31G, 6mm insulin syringe may suit small prescribed volumes. Follow your prescriber's device advice.
Can I use an insulin syringe for testosterone?
Yes, when your prescriber has instructed a subcutaneous workflow that suits an insulin syringe. The barrel is graduated in U-100 unit marks (100 unit marks = 1mL) for small-volume measurement.
Why does the oil draw so slowly?
Testosterone is an oil-based solution and fine needles draw it slowly. If your pharmacist says hand-warming is appropriate, hold the vial in clean hands briefly; do not heat it with hot water or a microwave, and return it to the labelled storage conditions after use.
Do I need a separate drawing needle for SubQ TRT?
Not usually for a single suitable vial in a fixed-needle insulin syringe workflow, but detachable draw needles are used in some prescribed setups. Never use the same fixed needle or syringe to draw from multiple stock vials, and follow pharmacist instructions for any multi-vial, ampoule, filter-needle, or transfer workflow. Gauge choice should come from your prescriber or pharmacist; repeated large-bore punctures can increase stopper damage or coring risk.
Related guides
- Subcutaneous Testosterone Injection Guide - full step-by-step technique, sites and troubleshooting.
- Needle Gauge Guide - how the gauge and length numbers work.
- How to Dispose of Syringes in Australia - sharps disposal done right.
- What Size Needle for GLP-1 Injections - the same question for semaglutide and tirzepatide.
- What Size Needle for Peptide Injections - the companion guide for reconstituted peptides.
This guide is about choosing injection supplies and is not medical advice. Follow your prescriber's dose, concentration and route, and keep up your regular TRT monitoring.