Halakhah sur Rois 2 7:3

When the possibility of curtailing even the brief span of life (ḥayyei sha'ah) which a terminal patient may anticipate is weighed against the possibility of cure accompanied by normal life expectancy, Jewish teaching accepts the principle that reasonable risks may be incurred in the hope of effecting a recovery. Thus, hazardous procedures are sanctioned in life-threatening situations even if the proposed therapy is such that the drug or procedure may prove to be not simply ineffective but deleterious in nature and the patient's life shortened thereby. This principle may be derived from the talmudic discussion in Avodah Zarah 27b concerning the incident of the four leprous men described in II Kings 7:3-4. The Syrian army had besieged Samaria. In addition, the region was suffering from a great famine. The lepers recognized that if they took no action they would die of hunger in a relatively short period of time. Were they, however, to cross into the Syrian lines one of two things would happen: either they would immediately be put to death as enemies or, if pitied because of their infirmity, they would be provided with food and their lives saved. Despite the danger, they reasoned, "Now therefore come, and let us fall into the host of the Syrians: if they save us alive, we shall live; and if they kill us we shall but die." The Gemara views this narrative not simply as a record of a historical event but as a paradigm providing scriptural sanction for assuming the risk of precipitating death in an attempt to restore conditions necessary for normal life-expectancy.