As a wildlife rehabilitator, vet medicine is some wild west bullshit. Fishmox is real, but you can buy a lot more than just that. One of the chemicals I see all the time is Dimethyl Sulfoxide, DMSO. It's an organic molecule that's exceedingly effective at penetrating tissues such as skin and taking other molecules with it. On it's own, it's pretty much harmless, but anything that gets mixed into DMSO will go right through your skin with it. Even contaminants from the air, or from other surfaces you touch. People put this stuff on animals with their bare hands, and some even swear by it as alternative medicine.