Insulin users would have better answers to this question, since they might inject multiple times a day, whereas GLP-1 users typically inject only weekly.

But in either case, the answer for subcutaneous injections using needles sized 29g and smaller is no.

No. So long as you rotate your injection site. My son is T1 and before his pump was getting 4-6 injections a day.

Yes, but only if you do it in the same spot every time.

To the skin? Probably not.

Heroin addicts and presumably anyone else who frequently injects into a vein can cause damage to the veins.

Seems completely negligible to the normal amount of tiny cuts and scrapes you accumulate on any given day.