The Hippocratic Oath says that a doctor should "never do harm" to patients--never "give a lethal drug to anyone if asked or advise such a plan." How then can a doctor justify assisted suicide or active euthanasia? And what worth is the oath if it can be compromised to benefit or comfort the patient in their final days? Is it time for more states to adopt the Death wih Dignity Act?The original Hippocratic Oath, from 2,500 years ago, did ban euthanasia. However, no version of it has been legal binding for thousands of years. It is not even clear that the oath that is reprinted all the time these days was actually from the Hippocratic school—it may have been Pythagorean. The text we use was rediscovered in the middle ages and has been used ever since to inspire doctors. These days it often appears in some watered down form in medical school graduation ceremonies. When people water it down, the first thing they do is remove the politically controversial stuff, like the bans on euthanasia or abortion.
In the modern world, there are no binding oaths for doctors, but there are codes of ethics. The first important one is the the Nurembuerg code, established in the wake of war crimes trials against Nazi doctors. Together with other international documents like the Geneva Declaration and the Helsinki declaration it is the basis of the doctrine of informed consent in international law and ethics.
In the US, the most important ethical code is probably the American Medical Association's code of ethics. The AMA opposes both euthanasia (by which they seem to mean active euthanasia) and physician assisted suicide. There isn't any real teeth to this, though. It is simply marked as an "opinion" of the AMA. As far as I know, members of the AMA who issue lethal prescriptions under the Death With Dignity Act in Oregon (and now Washington!) are not sanctioned in any way. I'm not even sure the AMA lobbied against either state's Death with Dignity Act.