I took it to this chatty thread in the General chat section of the forum because I didn't want to derail or scare the person who posted the other thread.
I've only used pens as well, but with the old syringes you pinch some skin and inject at an angle to inject subcutaneously.
I've done it wit a guinea pig needing medication, and with a goat too. The goat needed both a intramuscular injection and a subcutaneous one, I was instructed to use the same needle for both.
For the one in the muscle I just rammed it in the goat's neck muscle, for the subcutaneous one I pinched and injected at an angle.
Told the vet injecting the goat myself wouldn't be a problem at all, but that was before I saw the needle!
Still, I managed!
A goat's skin isn't much thicker than a human's, so if you can inject under the skin with this you definitely can with the ones in
@Grant_Vicat 's picture!