Critics regard Krebs as a quack.
Some sources incorrectly claim either Krebs or his father, Ernst T. Krebs Sr., or both as the discoverer(s) of laetrile, but the substance was actually first discovered in 1830 in France by two French chemists. In the presence of certain enzymes, amygdalin breaks down into glucose, benzaldehyde, and hydrogen cyanide (which is poisonous). It was tried as an anticancer agent in Germany in 1892, but was discarded as ineffective and too toxic for that purpose. Krebs' father was a San Francisco physician who happened upon amygdalin while looking for a substance which would improve the taste of bootleg whiskey. After learning of its previous use in Germany, he began to promote it as a cancer treatment. The son purified the amygdalin into laetrile, and continued his father's promotion of the product as a drug to cure cancer.