evidence: Do apricot seeds fight cancer? What is vitamin B17? The idea that such products could fight cancer arose because apricot pits contain a substance called amygdalin, which is broken down by the body to produce hydrogen cyanide. Because of the levels of certain enzymes in cancer cells, hydrogen cyanide is particularly poisonous to such cells, according to one theory. Another theory holds that cancer is a result of a vitamin deficiency, and amygdalin – also known as vitamin B17 – can restore the body to health. Yet another theory claims that in damaging cancer cells, amygdalin boosts the immune system to fight cancer.