laetrile lā´ətrĭl˝ [key], name given to the chemical amygdalin, a substance derived from an extract of the kernels of many fruits, notably apricots, bitter almonds, and peaches. The idea that laetrile might selectively destroy cancer cells was developed by Dr. Ernst T. Krebs, Sr., a German immigrant to the United States, in the 1920s and was later refined by his son Ernst T. Krebs, Jr., who used the name vitamin B 17 for amygdalin and the derivatives of amygdalin that he developed. The Krebses had hypothesized that an enzyme that was more abundant in tumor cells than normal cells acted on laetrile to produce cytotoxic cyanide. (Cyanide is naturally produced in the intestines when laetrile is acted upon by intestinal bacteria.) This hypothesis and several refinements of it were proved to be untrue, as was the younger Krebs's later classification of the substance as a vitamin.

The subject of controversy for many years, laetrile was subjected to much scientific scrutiny in the 1970s. Investigations showed that anecdotal reports of improvement with laetrile were insufficient proof of effectiveness. Clinical trials showed no effectiveness in shrinking tumors, prolonging survival, or improving the quality of the patient's life. Toxicity from cyanide poisoning in some patients, coupled with the drug's ineffectiveness, led the Food and Drug Administration to label laetrile a fraud. Interstate shipment and shipment from other countries are illegal, but it is still legal in some states and in Mexico.

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