In many circumstances, yes. Fraud and commercial speech are subject to more restrictions. If someone isn't holding themselves out in a commercial context as one

In many circumstances, yes. Fraud and commercial speech are subject to more restrictions. If someone isn't holding themselves out in a commercial context as one who is licensed to practice medicine, isn't actually practicing medicine, and isn't otherwise committing fraud, it's probably ok legally speaking. See United States v. Alvarez for a Supreme Court case that grapples with such issues. A man falsely claimed to be a veteran and a recipient of the Medal of Honor at a government meeting. He was prosecuted and convicted for this conduct. He appealed to the Supreme Court and won.