The dose makes the poison: dangerous plants

By Marieke Hendriksen

'The gift of chemistry', by P.H. Looff, 1773
‘The gift of chemistry’, by P.H. Looff, 1773

My current research focuses on how certain materials, particularly metals, gemstones and glass, mostly disappeared from the medicine of Boerhaave and his followers. This primarily had to do with how these substances were chemically understood. When I tell this to people, they mostly nod in agreement, assuming that Leiden professor of medicine, chemistry and botany Herman Boerhaave (1668-1738) discovered how poisonous metals are for the human body through chemical analysis, and subsequently banished them from his medicine. The truth is a little more nuanced. Boerhaave’s ideas about these substances and his idea of poisonousness were deeply influenced by those of illustrious medical men like Paracelsus (1493-1541) and the common alchemical understandings of matter at the time.

The reason that Boerhaave thought most metals were poisonous for humans for example was not because he had systematically tested the effects of ingestion, but because he argued from an alchemical understanding that the corrosive nature of most metals, except for gold and iron, meant that they were generally damaging for the human body. Only in very severe cases of otherwise incurable diseases, such as syphilis, could an appropriately cultivated preparation of a heavy metal like mercury be used as a last resource. Neither did Boerhaave’s discouragement of the use of substances such as metals in medicine mean that he thought all vegetable substances were safe to use, and he discusses vegetable poisons in his lectures.

Poisonous hemlock (Conium maculatum) Courtesy of  Kurt Stüber,
Poisonous hemlock (Conium maculatum) Courtesy of Kurt Stüber,

However, Boerhaave, although critical of Paracelsus, was also aware of the truth of his adage ‘the dose makes the poison,’ and a fervent advocate of critical thinking and practical research, who told his readers to think for themselves and undertake their own experiments. The impact of his work and attitude becomes clear from an array of eighteenth-century works on medicine, ranging from academic chemistry and medical handbooks to private recipe collections and small practical medical treatises in the vernacular. A nice example is a pamphlet by Philip Looff (1745-1778), a medical doctor from Groningen, published in 1773 under the title Donum Chemicum, literally ‘the gift of chemistry.’[1]

Rough chervil (Chaerophyllum temulentum). Couresy of Kurt Stüber,
Rough chervil (Chaerophyllum temulentum). Courtesy of Kurt Stüber,

Although Looff obviously never met Boerhaave, he dedicated his book to Boerhaave’s successor and former student H.D. Gaub (1705-1780), who endorsed it, and Looff repeatedly speaks of Boerhaave’s work admiringly in the book. Completely in Boerhaave’s line of thought too is his discussion of the medicinal use of rough chervil (‘dulle kervel’ or Chaerophyllum temulum), a slightly poisonous plant. It was often confused with hemlock  (Conium maculatum), which is said to have been the active ingredient in Socrates’ poisoned chalice. Like Boerhaave, Looff begins with a warning that any use of poisonous substances by the uniformed is extremely dangerous, and continuous to describe an observation of the usefulness of rough chervil in the cure of leprosy. According to Looff, he and his father in law, also a medical doctor, had successfully cured a woman suffering from leprosy be treating her with a very week solution of rough chervil and sarsaparilla – a handful of rough chervil and an ounce and a half of sarsaparilla boiled in 28 or 30 ounces of water for three hours. Although the patient went temporarily blind each time she took a two-ounce dose of the brew, after seven weeks of two daily doses, she was cured of her leprosy.

In hindsight it seems rather risky that Looff published his findings in the vernacular, as it is very easy to confuse rough chervil and hemlock – ingestion of only 6 to 8 leaves of fresh hemlock can be fatal. Yet however dangerous, these kinds of case studies were also essential for the study of the effects of potentially poisonous plants on the human body, and thus indirectly for the early nineteenth-century invention of isolating plant alkaloids  – the active chemical principles of plants that had been used medicinally for centuries and that we still know and use today.[2]


Plant prints: permission from Kurt Stüber,


[2] Swann, John P. “The Pharmaceutical Industries.” edited by Peter J. Bowler and John V. Pickstone, The Cambridge History of Science. Volume 6: Modern Life and Earth Sciences: 126–40. (The Cambridge History of Science, 2009), p. 127-8.

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