The active ingredients of Listerine are eucalyptol, menthol, methyl salicylate, and thymol. These four plant-derived chemicals also show up in Gold Bond Powder, and -- for what it's worth -- I've noticed that both products share a very similar smell. (Try comparing the scent of original Listerine, which is the yellow Listerine, with Gold Bond Powder.)
Here's a quick summary of these ingredients:
- Thymol is derived mainly from thyme essential oil. It is a proven antibacterial.
- Eucalyptol is derived mainly from eucalyptus essential oil. Just scanning through recent research, I see that it's been shown to have broad-range antibacterial effects somewhat like thymol (but perhaps not quite as strong). It also appears to be a deodorant.
- Methyl Salicylate is derived from essential oil of wintergreen. Not finding much research on this chemical. According to King's American Dispensatory (see the link for "wintergreen" just above), it is a stimulant, a taste-disguiser, and a local pain reliever. It's quite toxic to people, so I can only assume that it would kill bacteria, too.
- Menthol should be a familiar compound. It's the active ingredient in cough drops. Menthol is actually derived from the essential oil of peppermint; it's what gives that herb its distinct smell and its cold, tingly bite. It is pain-relieving, cooling, and -- according to King's American Dispensatory -- most likely antibacterial.
By my reckoning, you could probably use Listerine for other purposes besides cleaning your mouth. It was originally formulated as a surgical antiseptic, after all. Sore throat gargle? Disinfectant?
Of course, these ingredients, though all naturally-derived, are probably synthesized for use in commercial products, because it would take a lot of plant material to isolate each of them! So, are they technically natural ingredients, then?