Often, the only difference between a medicine and a poison is the dose. An aspirin, for example, will relieve pain. But 200 aspirins will kill you. Some substances are extremely toxic, and therefore, are primarily known as a poison. Yet, even poisons, like those listed below, can have medicinal value.

Snake Venom

Snake venom, collected from farmed reptiles, has been used to make antivenom for snake bites for decades. To produce antivenom, pure snake venom is diluted and then injected into mammals such as sheep, goats, rabbits and horses. After being injected, the animal undergoes an immune response, and scientists can collect the antibodies produced by the animal to make the antivenom.

But making drugs from snake venom is a relatively new idea. For example, the protein disintegron eristostatin, extracted from the venom of the Asian sand viper, has been shown to help people’s immune systems fight malignant melanoma. Previous studies have shown that disintegron eristostatin stops melanoma cells from colonizing in the liver and lungs of mice. And additional work is being done to investigate the role that snake venoms may play in inhibiting neurological disorders such as Parkinson’s disease, amyotrophic lateral sclerosis (ALS) and more.

Tetrodotoxin

An extremely potent neurotoxin, tetrodotoxin (TTX), derived from the pufferfish, is one of the most concentrated poisons known to man. TTX blocks voltage-gated sodium channels (VGSCs), which play a critical role in neuronal function. Administration of some specific TTX-sensitive VGSCs has shown improvement in cases of chronic pain. The administration of TTX at doses below those that produce acute toxic reactions in normal nerves has been used in humans and animals under different pain conditions. Various studies show value for the use of TTX as a potential therapeutic agent for pain. Further investigation of tetrodotoxin may lead to the use of this agent for the prevention of ischemic damage of the brain that follows stroke, suppressing pain in cancer patients, and relieving uncomfortable symptoms of withdrawal in opiate addicts.

Nightshade

Deadly Nightshade, or Atropa belladonna, has a long and rich history of use as a lethal poison. Over the centuries, Deadly Nightshade has been used for poisoning enemy troops in war, as an implement of torture, for its extreme hallucinogenic activity, and reportedly for witchcraft, applied as a vaginal ointment to produce a sense of flying.

Rich in the tropane alkaloids scopolamine and hyoscyamine, belladonna (which means beautiful woman) was used by women to dilate their pupils, to make them appear more alluring.

Medicine has found numerous uses for the toxins in Deadly Nightshade. Scopolamine is administered by eye surgeons to dilate pupils and to alleviate motion sickness. Hyoscyamine is employed in cases of irritable bowel syndrome, diverticulitis, colic and other inflammatory conditions of the GI tract. Belladonna toxins can help to control muscle spasms in cases of Parkinson's disease. When applied topically, belladonna is also quite effective in reducing pain, as in rheumatism, sciatica or neuralgia.

Hemlock

The association of hemlock (Conium maculatum ) with the death of Socrates has made this one of the most widely known botanicals in ancient medicine. In antiquity, people were familiar with hemlock and its poisonous applications. In hemlock, eight piperidine alkaloids have been identified. Two of these, in the highest concentrations, account for the toxicity of this plant. These two compounds are g-coniceine and coniine. The latter is approximately 8 times more toxic than the former.

As an ancient medicine, hemlock has primarily been used for its sedative and antispasmodic properties. The plant was also used by Greek and Arab physicians to treat joint pain. Coniine has uses in surgery for its antispasmodic properties, as it can prevent muscle twitching while “under the knife.” But great care must be taken when using hemlock toxins, and in cases of surgery the patient must be on a respirator. Otherwise the coniine will stop breathing.

Foxglove

Foxglove, or Digitalis purpurea, contains a group of compounds known collectively as Digitalin. These agents, also known as cardiac glycosides, are used to strengthen cardiac output, and as an anti-arrhythmic agent to control heart rate, notably in cases of atrial fibrillation.

As recently as 1998, Digoxin was approved to treat heart failure by the U.S. Food and Drug Administration. However, Digoxin use is declining, likely due to the fact that the drug cannot be patented, and offers little profit for pharmaceutical companies as compared with newer drugs under patent. Nonetheless, Digoxin works for a variety of cardiac needs, and has proven its medicinal value over time.