It depends on why you're mouth-breathing. If you're mouth-breathing because your sinuses are always clogged and it's hard to breathe through your nose because of it, then maybe. It's a antihistamine in that regard.
If it's being caused by quite literally anything else (and it probably is), then this wouldn't work. Most people mouth breathe because of a medical concern, such as a deviated septum, or because of posture / throat concerns / jaw is not aligned properly / it causes too much pain / etc.