Can one practice functional medicine (a.k.a. applied nutritional science, lifestyle medicine) with a ph.d. in chemistry?

About good medicine: Modern medicine in developed countries is based on good science. We are in a free society, so many people NOT practicing modern medicine do create other types of "medicine" practices. Because our society wants to best protect ourselves from more risky, less effective types of "medicine," only persons with mainstream medical degree should advertise a practice of medicine. We have advanced this far.

Check local laws: If you offered yourself as a medical practitioner in my state without a medical degree you could end up in jail. Some countries/ regions may have other approaches to what they permit you to advertise but I would investigate the legal landscape and the limits of what non-MD's can say about their offerings.