"Laetrile" is used interchangeably with "amygdalin" to designate natural substances, derived primarily from apricots and almonds, that can release cyanide, which is lethal to living organisms. In the 1920s, Dr. Ernst T. Krebs, Sr., formulated a theory that amygdalin could kill cancer cells. His theory was inconsistent with biochemical facts and has since been modified at least twice by his son, Ernst T. Krebs, Jr. Extensive work has been done by cancer scientists to test the claim that Laetrile fights cancer. Many animal experiments in the 1970s showed a complete lack of tumor killing by Laetrile. Reviews of the medical records of patients whose cancers were claimed to be reduced or cured after Laetrile treatment found insufficient medical evidence to judge Laetrile's efficacy. Finally, in a clinical trial in cancer patients reported in 1982, Laetrile neither caused shrinkage of tumors, nor increased survival time, nor alleviated cancer symptoms, nor enhanced well-being. Several reports in the medical literature document instances in which Laetrile has caused serious, life-threatening toxicity when taken in large doses in the manner prescribed by Laetrile advocates. In light of the lack of efficacy of Laetrile and its demonstrated ability to cause harm, Laetrile should not be used to treat cancer.