12 Toxic Substances That Have Been Used As Medicine

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Vote up the toxins you're most bewildered to see in medicine.

The history of medicine is long and full of bizarre concoctions. Humans have stopped at nothing to find solutions to their ailments, which has led to questionable experiments, poisons used as medicines, and other strange medical practices.

But to find treatments and cures, sometimes scientists have needed to experiment with dangerous and sometimes deadly compounds. You might be surprised to find that some toxic substances in history are still in use today.


  • Venom From Funnel Web Spiders Can Stop A 'Death Signal'
    Photo: Provaprova / Wikimedia Commons / Public domain

    The funnel web spider is native to eastern Australia. Not all 40 species of the spider are dangerous, but those that are eliminated at least 13 people before anti-venom was created in 1981. While most people want to steer clear of these arachnids, some scientists have looked into their venom as a potential treatment for strokes, heart attacks, and cancer. 

    Scientists have isolated a protein that reduced the amount of brain damage in strokes suffered by lab rats. When scientists tested the protein on human heart cells, the results were even better: The protein effectively blocked the “death signal” in a heart attack. That means the protein stopped the signal the heart receives to start cell death due to a lack of oxygen.  

    Scientists hope to create a synthetic protein out of the venom and make it available to first responders. If paramedics can administer the drug during the early stages of a heart attack, they might be able to save the patient from extensive heart damage. 

  • Mercury occurs in various forms, both organically and inorganically. It forms naturally in rock in the Earth's crust, including deposits of coal and other minerals. All exposure to mercury is toxic, and depending on the type of exposure, side effects vary. Generally speaking, mercury harms the nervous system, liver, kidneys, and immune system. 

    In the early 1900s, a type of inorganic mercury known as calomel (AKA sweet mercury) was used to treat illnesses including typhus and syphilis. It was also believed to be beneficial in helping calm the gums of babies who were teething. Persistent use of teething powders laced with mercury led to many babies being diagnosed with acrodynia (AKA pink disease). Pink disease causes a pink discoloration in the hands and feet of babies, along with sweating, swelling, and hypertension. 

    If left untreated, symptoms can worsen to include neuropsychiatric damage and negative impacts on physical and mental development. Scientists finally admitted that mercury was to blame for pink disease, and the product was banned in North America in the 1940s. Despite the risks, a few countries continue to use calomel in medical products. 

    The biggest source of human contact with mercury today is through fish. Nearly all fish contain traces of methylmercury. Human activity has led to large quantities of chemicals, including mercury, being poured into the oceans.  

  • Hemlock Was Used By The Ancient Greeks For Joint Pain
    Photo: Françoise Caclin / Wikimedia Commons / CC-BY-SA 2.0 FR

    Conium maculatum (AKA hemlock) goes by several names and grows throughout the United States. Despite its beauty, all parts of the plant are poisonous. Cattle and sheep are often unsuspecting victims of the plant, which can cause death after two to three hours due to respiratory paralysis. People can be easily poisoned if they consume any part of the plant, and reportedly, children who innocently created small whistles from the hollow stems of the hemlock plant have perished

    Hemlock has also been used throughout history to treat various ailments. It produces strong physiological effects that range in severity depending on how much is consumed. Physicians from ancient Greece, as well as the Middle East, used the plant for its sedative, antispasmodic, and painkiller qualities. Small amounts were administered to individuals suffering from illnesses including joint pain.

    The proper dose was critical because any miscalculation could take a person from sedation to complete respiratory distress. The invention of safer pain management alternatives has largely led to the discontinuation of hemlock.

  • Warfarin is a tasteless, odorless compound found naturally in sweet clover and similar plants. In the 1950s, when it was discovered that cattle got very ill and perished after eating sweet clover, people began using warfarin as a rodenticide. It causes vermin to hemorrhage and bleed out.

    Warfarin's anticoagulant properties make it dangerous, but this same toxicity makes it an effective medication in therapeutic doses. It's been used to treat deep-vein thrombosis and pulmonary embolism, and doctors have prescribed warfarin as a blood thinner since 1954. It’s become one of the most widely used thinners on the market, but people who use the medication must be monitored - significant bleeding can result if too much is taken. 

    Warfarin's use as a rodenticide has for the most part subsided.