Euthanasia is traditionally divided on "passive" and "active" euthanasia, based on a role of a physician in the process. "Passive" euthanasia refers to the ending of a patient's life by the deliberate withholding of drugs or other life-sustaining treatment on part of a physician. "Active" euthanasia refers to an act of injecting a patient with a lethal dose of medication with the intention of ending the patient’s life, at the patient’s request.
Both active and passive euthanasia ultimately lead to patient's death. The distinction between them is meaningless - there is no ethical difference between letting a patient die and administering a lethal dose of drugs since both happen under a physician's supervision. Moreover, a refusal of active euthanasia while permitting passive one might prolong suffering on a part of a patient, which is contrary to the whole idea of euthanasia, that is relief from incurable pain.
Two different actions (or omissions of actions) resulting in the same outcome are not necessarily ethically identical since reasons for the decision and circumstances might differ. Actively causing a patient's death is not identical with discontinuation of treatment, since the former constitutes a deliberate act of an external agent in a patient's organism, while the latter lets a natural process, already in motion, to run its course.
[P1] Both active and passive euthanasia result in a patient's death. [P2] In both cases, a physician is involved, either by an action or lack of it, in the process. [P3] Therefore, there is no reason to distinguish between the two practices.
Rejecting the premises
[Rejecting P3] There is no basis for treating an act of deliberate killing and an act of forgoing life-sustaining treatment as identical.