From Wikipedia..........
Those who possess a doctoral degree are generally entitled to call themselves "Doctor", although restrictions apply in some jurisdictions.
In the United Kingdom, Australia, New Zealand and other areas whose culture was recently linked to the United Kingdom, the title Doctor generally applies to both academic and clinical environment. "Registered medical practitioners" usually do not have a doctorate; rather, they have a Bachelor of Medicine. Cultural conventions exist, clinicians who are Members or Fellows of the Royal College of Surgeons are an exception. As an homage to their predecessors, the barber surgeons, they prefer to be addressed as Mr, Mrs, Ms or Miss, even if they do hold a doctorate. This is first because they have normally achieved another degree - that of Master of Surgery (MCh from the Latin magister chirurgiae) from a university. When a medically-qualified person passes the notoriously difficult examinations which enable them to become a member of one or more of the Royal Colleges of Surgeons and become "MRCS", it is customary for them to drop the "doctor" prefix and take up "mister". This rule applies to any doctor of any grade who has passed the appropriate exams, and is not the exclusive province of consultant-level surgeons. In recent times, other surgically-orientated specialists, such as gynaecologists, have also adopted the "mister" prefix. Physicians, on the other hand, when they pass their "MRCP" examinations, which enable them to become members of the Royal College of Physicians, do not drop the "Doctor" prefix and remain doctor, even when they are consultants. In the United Kingdom the status and rank of consultant surgeons with the MRCS, titled "mister", and consultant physicians with the MRCP, titled "doctor", is identical. Surgeons in the USA and elsewhere remain steadfastly "doctor".
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