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Tuesday 5th June 2012

An email from Dr *****, in Germany, tells me that he wrote his PhD about thujone and asks if I was aware that Thuja occidentalis, white cedar, also produces thujone. The essential oil is reported to contain up to 80% thujone so you have to wonder why, when oil of wormwood from Artemisia absinthium is said to contain 40-60% thujone there is much less information about the former.

I did a quick search of a database of research papers and found that the keywords ‘Thuja’ and ‘toxicity’ produced six results but ‘Artemisia’ and ‘toxicity’ gave 99 documents.

Though my correspondent didn’t mention it, Thuja plicata, western red cedar, also has an oil containing thujone, reportedly up to 87% in fact. The International Poisonous Plants Checklist has many more citations for this species than for the occidentalis. Most refer to contact dermatitis or asthma related problems whereas the papers cited under plants in the Artemisia genus are mostly concerned with poisoning, of humans and livestock, by ingestion.

In the 19th and early 20th century, Thuja was widely used for its alleged therapeutic properties though care should be taken when reading about these because it was a favourite of Hahnemann, the exploiter of homeopathy, and, therefore, it is claimed to be efficacious in a huge range of conditions. Even so, its potential to cause harm was well-known and well-documented.¹

My correspondent also developed a point that flows from what I wrote about the way plant constituents were named after the plant. He noted that this can lead to the same chemical being given different names because it occurs in different plants. He explained that 
tanacetone, found in Tanacetum vulgare, tansy, is essentially thujone. He also noted that tansy produces over ten so-called chemotypes, slight differences in the chemical composition of apparently identical plants.

From the very beginnings of my work with poisonous plants I have favoured using the botanical names because common names can get applied to completely different genera producing completely different substances. For example, ‘Jack in the pulpit’ may mean Arum maculatum or Arisaema triphyllum. Identifying a plant for medicinal use by its common name is, therefore, not to be recommended though a great many herbal preparation websites do use common names predominantly.

You might think that identifying plants by their appearance would be a better way. For some time, I’ve visited a gardening forum with a section for questions. Many of these questions are of the ‘What is this?’ variety. The site is a community of amateur gardeners but between them there are many hundreds of years of gardening experience. Nonetheless, a frequent answer to the question ‘What is this?’ is ‘Don’t know’.

Misidentification by sight is not a new problem I mentioned before that Thomas Johnson, the botanist who substantially revised John Gerard’s ‘Herbal’ for the 1633 second edition was concerned about apothecaries buying their plants from women who were not sufficiently skilled to always collect the correct plant.

Now, with today’s email, it seems you can’t always rely on correct identification of a plant being a way to be sure of what it contains. Actually, that’s not a completely new thought because I’ve noted before that Conium maculatum, poison hemlock, is said to vary in strength becoming stronger in warmer climates. In that same blog, I said;

‘use of a plant itself for medicinal purposes is a very stupid thing to do.’

The email Dr ***** was kind enough to write reinforces that opinion.

References

^1.A Treatise on Thuja Occidentalis.  Professor H. W. Felter 1904

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