Yup, stick that syringe right in your anus.
It's well known that the rectum absorbs incredibly efficiently.
Jk bro, there was a thread years ago here where some idiot who kept trying to inject in his ass, but every time he got close to his hole, it would pucker up and he couldn't bring himself to do it. Idiot didn't realize "in your ass" means in the glute muscle, not literally inside of your anus. It is a legendary thread here, so hilarious.