Remember when your mother told you never to eat the contents of a dented tin can? Well, she was absolutely right.A dent can create space for the proliferation of an extremely dangerous and toxic bacteria called clostridium botulinum, which will immediately contaminate the contents of a can, and if consumed even in tiny quantities, can make a person seriously ill.And yet that same lethal bacteria is also the basis for Botox, a drug most commonly known for its ability to effectively erase wrinkles and smooth the skin, as well as one that is also used as treatment for several medical conditions, including strabismus (an abnormal alignment of the eye), facial twitches, chronic neck and back pain and migraines.MORE: The Hard Facts Behind Food PoisoningWelcome to the Jekyll-and-Hyde world of poisons, a paradoxical and contradictory universe of harming and healing that throughout history has both terrified and transfixed humankind.Names like cyanide and arsenic—poisons that have killed off many-a-character in Agatha Christie mysteries—send shivers down our spines, along with snake venom and the highly toxic nature of many plants. But as lethal as poisons may be, many can, if used in the appropriate way and with the correct dosage, be extremely therapeutic, and they are an integral component of numerous medicines that are in common use today.“The Greeks had two basic words for poison: ‘Toxikon,’ when applied to an arrow or spear, and ‘Pharmakon,’ that which can be eaten or used for healing,” explains Mark Siddall, curator of “The Power of Poison,” an exciting exhibition that debuts at New York’s American Museum of Natural History on November 16, 2013. “There is an interesting history between the overlap of poisons as poison and poisons as medicine. Both, in essence, are interfering with some sort of physiological process, so they are doing the same thing, only the drug is interfering with something we want it to interfere with, whereas the poison is interfering with something we don’t want interfered with.”That means that as much as a poison may destroy the human body, it can, if dosed properly, also heal and repair it.MORE: Arsenic in the Food Supply: Rice. Juice. Where Else?Take Botox, for example. A full-blown dose of clostridium botulinum would wreak serious neurological damage on the body and can even result in death, explains Robert Schwarcz, M.D., a New York-based board-certified cosmetic and reconstructive plastic surgeon, but when used in minute doses, the toxin safely relaxes facial muscles, giving the skin a more youthful, wrinkle-free appearance. “The toxin we inject for cosmetic purposes is such a minute amount that it only acts locally,” Schwarcz says.For scientists, experimenting with deadly poisons to make a therapeutic dose has everything to do with balancing ED 50 (the effective dose of a particular toxin at which 50 percent of recipients will achieve the desired benefit) against LD 50 (the dosage at which 50 percent of the recipients are expected to perish), notes Siddall.“If both of these are too close together, it is very easy to overdose and cause toxicity, so those drugs that are most useful are those with an ED 50 that is relatively small compared to LD 50,” he says.There are poisons in just about every aspect of the natural world. They’re found in plants as well as in insects and reptiles. Human beings, Siddall says, haven’t even uncovered the myriad poisons in the environment and all of their potential therapeutic properties, but continued research into poisons, toxins and venoms reveals surprising results. Case in point: One of the properties of the venom of the poisonous snake genus Bothrops causes blood pressure to drop and can be used in blood pressure medication.But the majority of toxins that are used in medicine today come from plants. Many plants are toxic in self-defense, Siddall says, based on the principle that “if it can’t run away, it probably has something in its own defense.” Both indigenous medicine and modern pharmacology have been able to make positive use of many plant toxins.MORE: Nature’s Medicine Cabinet: Medicinal PlantsIn the following slideshow, Siddall and Peter Macinnis, an Australian science writer and author of “Poisons: From Hemlock to Botox to the Killer Bean of Calabar,” share their insight into the most common plant toxins and their use in medicines. Click through to see more.