Most of us have childhood memories of running through patches of tall grass and suddenly feeling a sharp pain as we scrape past a patch of nettles. The next few minutes would be spent frantically searching for a dock leaf to stop the pain, and for many of us this would still be our first thought, but is it true that these leaves numb the pain of a nettle sting, or is it just a myth with a placebo-type effect?
With its distinctively green serrated-edged leaves, the urtica dioica is the most common nettle found in the UK.1 The stem and underside of its leaves are covered in tiny hollow hairs called trichomes, a defensive tool to protect the plant from herbivores.
These trichomes have very fragile tips made of silica which can easily be broken. When your skin comes into contact with the leaves, the tips of the trichomes break off and the remaining sharp structure pierces the skin. Due to the hollow nature of the hair, breaking off the tip creates pressure and forces the chemical contents out of the hair and into the skin.
Image adapted from reference 3.
Within the hair itself is a mixture of chemicals including histamine, acetylcholine, serotonin and formic acid. It was previously thought that formic acid was the main contributor to the stinging as it’s known to cause skin burns, but this is unlikely as the concentration of formic acid in the trichomes of the urtica dioica is far too low to cause such a lengthy stinging sensation. It is now believed that histamine, acetylcholine and serotonin are the main causes of this pain.
The histamine in this chemical mixture is responsible for the swelling and itching of the skin following a sting, whilst acetylcholine and serotonin are thought to cause skin irritation and the subsequent pain.2
Interestingly, male and female nettles are distinguishable by the density of their stinging hairs. Since female plants are responsible for producing seeds, they have evolved to protect their reproductive organs by producing more trichomes on their stalks and leaves compared with male plants.3
The biggest question associated with nettle stings is ‘do dock leaves really help?’. Many have suggested that dock leaves are alkaline in nature and therefore neutralise the formic acid present after the sting but this statement has since been challenged. The sap of the dock leaf is also acidic, meaning neutralisation would not possible. Another theory is that when the dock leaf is rubbed on the site of the sting, the sap deposits on the skins surface where it quickly evaporates, causing a cooling sensation that may provide some relief.4 Although there is no scientific evidence dock leaves help, many still swear by them after a nettle sting. The soothing effect may be all in our head, but if it works why change it?!
References
- Plants For A Future. Urtica dioica – L. [Online]. 2006. Available from: https://pfaf.org/USER/Plant.aspx?LatinName=Urtica+dioica
- Melissa Petruzzello. Stinging Nettle. [Online]. 2018. Available from: https://www.britannica.com/plant/stinging-nettle
- In Defence of Plants. The Stinging Nettles. [Online]. 2017. Available from: http://www.indefenseofplants.com/blog/2017/7/6/the-stinging-nettles
- Kat Arney, The Naked Scientists. Do Dock Leaves Stop Stinging Nettle Stings?. [Online]. 2015. Available from: https://www.thenakedscientists.com/articles/interviews/do-doc-leaves-stop-stinging
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