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Water, salt, oxygen and every medicine in your home can harm you in a large enough amount. Toxicologists sum this up as "the dose makes the poison", and they credit the idea to a sixteenth-century physician called Paracelsus. This post covers who he was, what he actually wrote, and how to work with doses per kilogram, the way the idea is used today.
Who was Paracelsus?
A 2021 historical review in Toxicology Reports by Michaleas and colleagues gives his life. He was born Theophrastus Philippus Aureolus Bombastus von Hohenheim in Einsiedeln, Switzerland, in November or December 1493. His father, a physician, moved the family to Villach in Austria in 1502. Working in mines owned by the Fugger family, the young Theophrastus saw miners' diseases, such as lung damage, and the harm caused by breathing mercury and arsenic fumes.
He probably took his medical doctorate at Ferrara in 1515 or 1516 and around then adopted the name Paracelsus, meaning "better than Celsus", a Roman medical writer. He travelled widely, and in 1526 was made town physician of Basel, with a post at the university. There he lectured in German instead of Latin, attacked the medicine of his day, and publicly burnt the books of Galen and Avicenna. He had to leave Basel in 1528 and died in Salzburg on 24 September 1541.
What did he actually write about dose?
Paracelsus used minerals, especially mercury, as medicines, and critics accused him of giving patients poisons. In the third of his written defences he answered them. The review quotes him, in English translation:
"What is there that is not a poison? All things are poison, and nothing is without poison. Solely the dose determines that a thing is not a poison."
The original was in German, so the exact English wording varies between translations. A detailed study of the text, What is there that is not poison? A study of the Third Defense by Paracelsus, appeared in Archives of Toxicology in 1986.
Why does toxicology still cite him?
Philippe Grandjean, in Paracelsus Revisited: The Dose Concept in a Complex World, writes that Paracelsus paved the way for the modern idea of a threshold and of a "no-adverse effect level": a dose below which no harm is seen. Grandjean also warns that the idea needs updating. Toxicology now has to deal with exposures during development, people who are unusually sensitive, and many chemicals acting together.
The review adds that Paracelsus thought each chemical has its greatest effect on a particular organ, an early form of what is now called target-organ toxicity.
A worked example: dose per kilogram
Doses are often compared per kilogram of body mass. The numbers below are made up for practice only and describe no real substance or medicine.
Suppose a test animal shows no harm below 50 mg of substance X per kg of body mass.
For a 60 kg adult, the same per-kilogram level is 50 × 60 = 3,000 mg, or 3 g.
For a 15 kg child, it is 50 × 15 = 750 mg.
Now give both people the same 600 mg amount. The adult receives 600 ÷ 60 = 10 mg/kg. The child receives 600 ÷ 15 = 40 mg/kg, four times as much per kilogram.
This is why medicine labels and doctors set children's doses separately. Never work out a real dose yourself; ask a doctor or pharmacist.
Common mistakes to avoid:
Thinking "natural" means safe. Paracelsus's point applies to every substance.
Comparing total amounts instead of mg per kg. Body size changes the dose that reaches the tissues.
Treating dose as the whole story. As Grandjean notes, timing and sensitivity matter too.
How does this connect to pesticides?
A chemical can also reach a dangerous dose indirectly. Some pesticides build up from one level of a food chain to the next, so a predator receives a far larger dose than was ever sprayed. Our post on Rachel Carson and Silent Spring works through the numbers. The chemist behind DDT designed it to last, which turned out to matter, as our post on Paul Müller and DDT explains.
What can a student take from Paracelsus's way of working?
The review stresses that Paracelsus preferred observing nature to repeating ancient texts, and learned from what he saw in mines and on battlefields. He was not always right: mercury, his favourite remedy, is now handled as a serious poison. But his habit of asking "how much?" instead of "is it good or bad?" is the right question in chemistry and biology.
When a textbook calls something toxic or safe, ask at what dose and for whom. If units like mg per kg trip you up, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning.
Next step: redo the worked example for a 45 kg teenager and compare the mg per kg with the adult and child values.
