According to the famous toxicologist Paracelsus, "the dose makes the poison." In other words, every chemical can be considered a poison if you take enough of it. Some chemicals, like water and iron, are necessary for life but toxic in the right amounts. Other chemicals are so dangerous they are simply considered poisons. Many poisons have therapeutic uses, yet a few have gained favored status for committing murders and suicides. Here are some notable examples.

Belladonna or Deadly Nightshade

Belladonna (Atropa belladona) gets its name from the Italian words bella donna for "beautiful lady" because the plant was a popular cosmetic in the Middle Ages. The juice of the berries could be used as a blush (probably not a good choice for lip stain). Diluting extracts from the plant in water made eye drops to dilate the pupils, making a lady appear attracted to her suitor (an effect that occurs naturally when a person is in love).

Another name for the plant is deadly nightshade, with good reason. The plant is high in toxic chemicals solanine, hyoscine (scopolamine), and atropine. Juice from the plant or its berries was used to tip arrows with poison. Eating a single leaf or eating 10 of the berries can cause death, although there is a report of one person who ate about 25 berries and lived to tell the tale.

Legend has it, Macbeth used deadly nightshade to poison Danes invading Scotland in 1040. There's evidence that the serial killer Locusta may have used nightshade to kill the Roman emperor Claudius, under contract with Agrippina the Younger. There are few confirmed cases of accidental deaths from deadly nightshade, but there are common plants related to Belladonna that can make you sick. For example, it's possible to get solanine poisoning from potatoes.

Asp Venom

Snake venom is an unpleasant poison for suicide and a dangerous murder weapon because, in order to use it, it's necessary to extract the poison from a venomous snake. Probably the most famous alleged use of snake venom is Cleopatra's suicide. Modern historians are unsure whether Cleopatra committed suicide or was murdered, plus there is evidence that a toxic salve might have caused her death rather than a snake.

If Cleopatra was indeed bitten by an asp, it wouldn't have been a quick and painless death. An asp is another name for an Egyptian cobra, a snake with which Cleopatra would have been familiar. She would have known the bite of the snake is extremely painful, but not always lethal. Cobra venom contains neurotoxins and cytotoxins. The bite site becomes painful, blistered, and swollen, while the venom leads to paralysis, headache, nausea, and convulsions. Death, if it occurs, is from respiratory failure... but that's only in its later stages, once it's had time to work on the lungs and heart. However the actual event went down, it's unlikely Shakespeare got it right.

Poison Hemlock

Poison hemlock (Conium maculatum) is a tall flowering plant with roots resembling carrots. All parts of the plant are rich in toxic alkaloids, which can cause paralysis and death from respiratory failure. Near the end, a victim of hemlock poisoning can't move, yet remains aware of his surroundings.

The most famous case of hemlock poisoning is the death of the Greek philosopher Socrates. He was found guilty of heresy and sentenced to drink hemlock, by his own hand. According to Plato's "Phaedo," Socrates drank the poison, walked a bit, then noticed his legs felt heavy. He lay on his back, reporting a lack of sensation and chill moving upward from his feet. Eventually, the poison reached his heart and he died.

Strychnine

The poison strychnine comes from seeds of the plant Strychnos nux vomica. The chemists who first isolated the toxin also obtained quinine from the same source, which was used to treat malaria. Like the alkaloids in hemlock and belladonna, strychnine causes paralysis that kills via respiratory failure. There's no antidote for the poison.

A famous historical account of strychnine poisoning is the case of Dr. Thomas Neil Cream. Starting in 1878, Cream killed at least seven women and one man — patients of his. After serving ten years in an American prison, Cream returned to London, where he poisoned more people. He was finally executed for murder in 1892.

Strychnine has been a common active ingredient in rat poison, but since there's no antidote, it has largely been replaced by safer toxins. This has been part of an ongoing effort to protect children and pets from accidental poisoning. Low doses of strychnine can be found in street drugs, where the compound acts as a mild hallucinogen. A very diluted form of the compound acts as a performance enhancer for athletes.

Arsenic

Arsenic is a metalloid element that kills by inhibiting enzyme production. It's found naturally throughout the environment, including foods. It's also used in certain common products, including pesticides and pressure-treated wood. Arsenic and its compounds were a popular poison in the Middle Ages because it was easy to obtain and the symptoms of arsenic poisoning (diarrhea, confusion, vomiting) resembled those of cholera. This made murder easy to suspect, yet difficult to prove.

The Borgia family was known to use arsenic to kill off rivals and enemies. Lucrezia Borgia, in particular, was reputed to be a skilled poisoner. While it's certain the family used poison, many of the accusations against Lucrezia appear to have been false. Famous people who have died from arsenic poisoning include Napoleon Bonaparte, George III of England, and Simon Bolivar.

Arsenic is not a good murder weapon choice in modern society because it's easy to detect now.

Polonium

Polonium, like arsenic, is a chemical element. Unlike arsenic, it's highly radioactive. If inhaled or ingested, it can kill in extremely low doses. It's estimated a single gram of vaporized polonium could kill over a million people. The poison doesn't kill immediately. Rather, the victim suffers headaches, diarrhea, hair loss, and other symptoms of radiation poisoning. There is no cure, with death occurring within days or weeks.

The most famous case of polonium poisoning was the use of polonium-210 to murder spy Alexander Litvinenko, who drank the radioactive material in a cup of green tea. It took him three weeks to die. It's believed Irene Curie, Marie and Pierre Curie's daughter, likely died from cancer that developed after a vial of polonium broke in her lab.