The Stanford Encyclopedia of Philosophy, in its July 28, 2004 entry titled "Doctrine of Double Effect," explained:
"The doctrine (or principle) of double effect is often invoked to explain the permissibility of an action that causes a serious harm, such as the death of a human being, as a side effect of promoting some good end. It is claimed that sometimes it is permissible to cause such a harm as a side effect (or 'double effect') of bringing about a good result even though it would not be permissible to cause such a harm as a means to bringing about the same good end. This reasoning is summarized with the claim that sometimes it is permissible to bring about as a merely foreseen side effect a harmful event that it would be impermissible to bring about intentionally...
A doctor who intends to hasten the death of a terminally ill patient by injecting a large dose of morphine would act impermissibly because he intends to bring about the patient's death. However, a doctor who intended to relieve the patient's pain with that same dose and merely foresaw the hastening of the patient's death would act permissibly."
The British Broadcasting Corporation (BBC) wrote in the Religion & Ethics section of its website, in an entry titled "The Doctrine of Double Effect" (accessed Nov. 15, 2006):
"This doctrine [of double effect] says that if doing something morally good has a morally bad side-effect it's ethically OK to do it providing the bad side-effect wasn't intended. This is true even if you foresaw that the bad effect would probably happen.
The principle is used to justify the case where a doctor gives drugs to a patient to relieve distressing symptoms even though he knows doing this may shorten the patient's life.
This is because the doctor is not aiming directly at killing the patient - the bad result of the patient's death is a side-effect of the good result of reducing the patient's pain.
Many doctors use this doctrine to justify the use of high doses of drugs such as morphine for the purpose of relieving suffering in terminally-ill patients even though they know the drugs are likely to cause the patient to die sooner.
Factors involved in the doctrine of double effect:
The good result must be achieved independently of the bad one: For the doctrine to apply, the bad result must not be the means of achieving the good one. So if the only way the drug relieves the patient's pain is by killing him, the doctrine of double effect doesn't apply.
The action must be proportional to the cause: If I give a patient a dose of drugs so large that it is certain to kill them, and that is also far greater than the dose needed to control their pain, I can't use the Doctrine of Double Effect to say that what I did was right.
The action must be appropriate (a): I also have to give the patient the right medicine. If I give the patient a fatal dose of pain-killing drugs, it's no use saying that my intention was to relieve their symptoms of vomiting if the drug doesn't have any effect on vomiting.
The action must be appropriate (b): I also have to give the patient the right medicine for their symptoms. If I give the patient a fatal dose of pain-killing drugs, it's no use saying that my intention was to relieve their symptoms of pain if the patient wasn't suffering from pain but from breathlessness.
The patient must be in a terminal condition: If I give the patient a fatal dose of pain-killing drugs and they would have recovered from their disease or injury if I hadn't given them the drugs, it's no use saying that my intention was to relieve their pain. And that applies even if there was no other way of controlling their pain."