Injecting into muscle
An intramuscular injection goes through the fat into the muscle, so it needs a longer needle than a subcutaneous one. Draw up with a wide blunt needle and swap to a sharp one to inject. The site, the length and the dose come from your prescriber.
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.
Why there are two needles
Intramuscular injections are usually oils or thicker solutions, and they go into a vial with a rubber stopper. Both of those argue for a different needle at each stage.
Drawing up through a fine needle is slow, and doing it repeatedly through a rubber stopper blunts the tip and cores small pieces of rubber into the vial. A drawing-up needle is wide and usually blunt, so it fills the barrel quickly and cannot be injected with by accident. Some are sharp fill needles for going through a stopper, which is a different thing again.
You then swap to a sharp needle of the right gauge and length to inject. One vial, two needles, both into the sharps container afterwards.
What the length is doing
A subcutaneous needle is short on purpose so it stops in the fat. An intramuscular needle has to pass through that fat and reach the muscle underneath, so it is longer. Common lengths run from about 25mm to 38mm.
How much fat the needle has to cross varies between people and between sites, which is exactly why the length is chosen for you rather than picked off a shelf. Too short and the medicine lands in fat, where an oil in particular absorbs badly and can sit as a painful lump. Too long is its own problem.
This is not a judgement you should make from a website, and we are not going to pretend otherwise. Your prescriber picks it.
Practical notes on the equipment
Luer-lock rather than luer-slip for anything viscous. A slip tip is a push fit, and pushing a thick oil generates enough pressure to separate the needle from the barrel. A lock screws on and cannot.
Warming an oil in your hands for a minute makes it noticeably easier to push. Do not heat it any other way.
Draw slowly. Pulling hard on a small syringe against a thick liquid is how you get a barrel full of bubbles that then take longer to clear than the slow draw would have taken.
Questions about this
If yours is not here, the contact form routes to the person who can answer it.