Is sweet woodruff poisonous?

Sweet Woodruff


Posts about sweet woodruff are often accompanied by toxicity warnings and advice not to take it with warfarin or to check with your doctor first. So I thought an article on coumarins would be useful. 

Sweet woodruff contains natural compounds called coumarins – like many other plants, including strawberries, apricots, blackcurrants, tea as well as hay, sweet clover, bison grass, meadowsweet and tonka beans. In fact the highest concentrations in the food chain are found in cassia bark, one of the 4 species of commercial cinnamon. Coumarins have a lovely vanilla like scent and taste. Dried sweet woodruff infused into milk makes wonderful desserts. Sweet woodruff cheesecake, pannacotta, ice cream, custard, kulfi… and is also infused to make the traditional Maywein.

Contraindications

Unmodified coumarins do not affect the action of warfarin or similar anticoagulant, blood thinning drugs. This is because coumarins do not deplete vitamin K which affects the blood coagulation system. Modified coumarin – such as dicoumarol or coumadin – is different. Dicoumarol is synthesised and known as warfarin (Coumadin) and is used as a. rat poison. In nature coumarin can be modified by certain fungal infections which occur if the plants are are harvested while wet or damp. Sweet woodruff picked on a dry day and properly dried without any mould developing will not contain modified coumarins. 

Cancer risk

Coumarin is not classed as carcinogenic.

Liver toxicity

In 1954, the American FDA banned coumarin as a food flavour additive. This was largely based on the results of experiments with lab rats. However, rats process coumarin in their livers in a different way to humans. Rat livers metabolise coumarin into 3,4-coumarin epoxide which is highly toxic. Human livers metabolise coumarin into 7-hydroxycoumarin which is far less harmful. 

The lethal dose of coumarin has been has been calculated at 293 mg per kilo of body weight (taken orally in rat trials). So a dose of 20.51 grams for a 70kg adult. Cassia cinnamon – the highest food source of coumarin – contains 5.8 to 12.1 mg of coumarin per teaspoon of powdered bark. So you’d need to consume between 1,700 and 4,100 teaspoons of cassia cinnamon powder to reach a lethal dose. The German Federal Institute for Risk Assessment (BFR) calculated that a tolerable amount – if taken daily – is 0.1 mg/kg but sees no danger in higher amounts being consumed occasionally for a short time. For a 70 kg person that’s a tolerable daily dose of 7 mg – about half to one rounded teaspoonful of cassia cinnamon powder.  

Sweet woodruff has a fresh to dry weight ratio of around 10:1. The coumarin content ranges from undetectable to 0.6% (Chevallier, 1996) which would mean there are 6 grams of coumarin per kilo of woodruff. So you would need to consume 3.41 kilos of sweet woodruff to reach the lethal dose – about 34 salad bags full (based on a 100g supermarket salad). It’s a tiny delicate plant and 3.41 kilos would need some very dedicated harvesting! 

Given that you don’t need more than a couple of sprigs at a time – a handful at the very most – it appears almost impossible to kill yourself with sweet woodruff. However, if you steep a lot of it in alcohol and paint the town green, you may well give yourself one hell of a hangover!

Sweet Woodruff
Old herbalists called it Woodderowffe and it made a lovely children’s rhyme

W OO DD E

RO W FF E

Recited as

Double ewe double oh double dee ee, 
are-oh double ewe double eff ee.

What do you think?

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