James Rachels, PhD, Professor at the University of Alabama at Birmingham, in his 1975 article "Active and Passive Euthanasia" published in The New England Journal of Medicine, argued:

“The distinction between active and passive euthanasia is thought to be crucial for medical ethics… However, a strong case can be made against this doctrine…

If one simply withholds treatment, it may take the patient longer to die, and so he may suffer more than he would if more direct action were taken and a lethal injection given. This fact provides strong reason for thinking that, once the initial decision not to prolong his agony has been made active euthanasia is actually preferable to passive euthanasia, rather than the reverse. To say otherwise is to endorse the option that leads to more suffering rather than less, and is contrary to the humanitarian impulse that prompts the decision not to prolong his life in the first place.

It is not exactly correct to say that in passive euthanasia the doctor does nothing, for he does do one thing that is very important: he lets the patient die. ‘Letting someone die’ is certainly different, in some respects, from other types of action – mainly in that it is a kind of action that one may perform by way of not performing certain other actions. For example, one may let a patient die by way of not giving medication, just as one may insult someone by way of not shaking his hand. But for any purpose of moral assessment, it is a type of action nonetheless. The decision to let a patient die is subject to moral appraisal in the same way that decision to kill him would be subject to moral appraisal. “