Err, they're just as likely to have a bad reaction to a salve -- they can just lick it off of themselves. I've never had a rat have any sort of "bad reaction" to a rat approved medication.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry