Boise Ed submitted this Argyle Sweater as a “total CIDU”. The gag must have something to do with the source of the material in the hypodermic needle, but the apparently obvious anatomical pun doesn’t seem to be present, and wouldn’t automatically qualify as an effective joke, either. There must be a better explanation.
Comics I Don't Understand 
He’s injecting her head with buttocks. And to help explain the stupidity of the misunderstanding, he’s not even a doctor.
I do recall hearing something about liposuction “product” being used for anti-wrinkle filler (same patient I suppose!).
I think David has the key here. It’s punning “buttocks”with “Botox.” The concept of injecting buttocks into her forehead never occurred to me. Argyle is often pretty funny (to me). This could be the least funny one I’ve ever seen.
Maybe it’s funny (as a secondary gag) that the, um, practitioner has the aplomb or non-self-awareness to be surprised at being addressed as “Doctor”.
Or it is an illegal operation and he is NOT a doctor.