In palliative care, as in most clinical care, there are occasions when the beneficial effects of certain therapies are accompanied by less desirable, even potentially lethal, effects.
This presents the clinician with a challenge to evaluate the permissibility of acting when one's otherwise legitimate act (i.e., relieving a terminally ill patient's pain) may also cause an effect one would normally be obliged to avoid (i.e., the patient's death).

The double effect criteria state that an action having foreseen harmful effects virtually inseparable from the good effect is justifiable upon satisfaction of the following:
- the nature of the act is itself good, or at least morally neutral;
- the clinician intends the good effect and not the bad, either as a means to the good or as an end itself;
- the good effect outweighs the bad effect in circumstances sufficiently grave to justify causing the bad effect, and the agent exercises due diligence to minimize the harm.

Some examples of it are easy for the general public to understand and accept.
Constipation is an unpleasant but acceptable adverse effect of codeine successfully controlling pain. On the other hand, drowsiness and, on rare occasions, depression of respiration in the patient on strong opioids may be difficult to accept, or may be seen as a means of abbreviating that patient's life. The control of pain was desired whilst the other effects were not; it is the responsibly of the clinician to weight the benefits against the other effects and explain that reasoning to relatives.

Randall F, Downie RS. Palliative care ethics: a companion for all specialties. Oxford, London and New York. Oxford University Press 1999. pp. 120-121.