Ethics is the branch of philosophy that deals with questions like this. This question is addressed by medical ethics under the name "the doctrine of double effect."

Here's the idea: A patient who is known to be near the end of life is on hospice care, and is suffering. Certain medications, most usually narcotics like morphine, have the power to ease the suffering of the dying patient. The medications are used with intent for this purpose, to ease the suffering of the dying. That is effect #1, and it is the intended effect. It is beneficial to ease a patient's suffering; medical ethicists approve of medical actions that are beneficial (that principle is called 'benificence.')

As a side effect, these medications can depress respiratory function, possibly to the point of hastening death. This is not an intended effect; it is effect #2. Because the medication was not prescribed for the purpose of bringing this about, the prescription was not harmful, and does not violate the "do no harm" principle (which ethicists call 'non-maleficence.')

Physician assisted suicide involves a prescription being given with the express intent to bring about the patient's death. This violates the principle of non-maleficence, which is a very ancient and well-established principle of medical ethics; and therefore many physicians find themselves in opposition to assisted suicide, when those same physicians have no problem using narcotics in a hospice situation.