Legally, the term "doctor" is not regulated, therefore, anyone can use the term, even if they don't hold a doctorate. Ethically, however, it is generally acceptable to use the term "doctor" as long as the individual received the degree from an accredited institution. Certain professions have restrictions regarding the usage of the term doctor. For example, the Behavioral Health Board in Indiana requires licensed practitioners under its authority to only use the term doctor if the doctoral degree was granted from a state recognized institution. In a sense, trumping the Dr. of Crystal Healing degrees.
Interesting article:
http://www.associatepublisher.com/e/d/do/doctor_(title).htm
I think it is important to understand what the term "doctor" means both explicitly and implicitly. Explicitly, doctor refers to someone who has received a doctorate degree, either an academic PhD or a professional doctorate (i.e. MD, PsyD, PharmD, etc.). Implicitly, however, when one refers to themselves as "doctor", the average person will assume that person is a medical doctor. It is important to use the title appropriately, and legally, most states will require an individual to have a doctorate degree in order to use the term. However, realizing that most people automatically assume "doctors" are medical doctors, it is important to use the term with caution. Whether this is correct or not doesn't matter. PhD's are the original "doctors" but the term has transcended into being generically attached to MD's. So if someone yells, "Is there a doctor on the plane?" and you are a PhD, you should not respond, even though you are in fact a doctor, as we know the person is actually referring to a physician. Additionally, if you are a nurse with a PhD, you probably shouldn't go around a hospital calling yourself "doctor"; why, because people will assume you are a medical doctor. We can argue what is accurate all day, but in the end, it is really about what the perception of the term is, whether that perception is correct or incorrect. If the public assumes that the term doctor is = to physician than it is important to use the term conservatively in order to not unintentionally deceive the individual. We have to consider the setting we are working in. When I have a PhD, and am referred to as doctor, if I walk into a hospital and introduce myself as doctor it will be assumed I am a physician. Similarly, if someone asks me what I do and I state ,I am a doctor, while this might be technically correct, it will be assumed I am a medical doctor. Consider the perception of the term and use it correctly.
There are other terms that have similar conflicts. For example, the term therapist denotes many different professionals. (i.e. physical therapist, psychotherapist, speech therapist, etc.); however, therapist has (like doctor) been generically applied to psychotherapists. If one is a physical therapist and states, I am a therapist the average person will probably assume the individual is a psychotherapist. Therefore, the term should be used with caution, as it can refer to many different professionals, but has been generically applied to a particular profession.
