I could have sworn I answered this question, but maybe it got deleted.
I tried injectable l-carnitine but got no results. However, you can buy human grade injectable l-carnitine from Synthetek
https://synthetek.com/en/product/synthetine-lipid-fat-transporter/
They're in Australia so it'll take awhile to get to you, but it's 100% legit if you want to give it a try. I think injectable acetyl-l-carnitine would be better, but I've never seen it.