The Hippocratic Oath says a number of things, not all of which doctors swear to today (for example, the Hippocratic Oath forbids any kind of surgery). Perhaps you are thinking of the most well-known part of the Hippocratic Oath, that forbids harm. This is regularly interpreted as forbidding "unnecessary harm," e.g. vaccinations hurt, but they are a necessary harm to prevent a greater harm. Cosmetic surgery involves harm to the patient--the pain of surgery and the risks of surgery--so the question is, does it prevent a greater harm? The right people to ask about this is the people who undergo cosmetic surgery--both the ones with successful and the ones with unsuccessful results. They should be fully informed so as to balance the potential harms and benefits for themselves.
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How justified is a doctor's decision to practise cosmetics when he had sworn upon the Hippocratic Oath?