Anyway, what about "a shot in the arm"? I always thought of that like getting punched , or maybe poisoned, but a commercial on the radio uses it as if you're getting medicine

I've always associated the phrase with the idea of vaccinations, like the TB vaccinations that used to be given to all 13 year-olds here in NZ - a literal shot in the arm. Here at least, the phrase conveys the idea of a boost, a pick-me-up.