Is peppermint extract ok to give dogs?

My 2 y/o German Shepherd has the most heinous, fishy breath and I was wondering if I can give him peppermint extract to deter it? He's got food allergies, specifically wheat and beef so I can't really mix it into anything like a homemade treat, so is it safe to give it to him straight, like on his tongue or should I try in water or something like that?