The Chemistry of Stinging Nettles 2016

Click to enlarge

Doubtless the majority of people reading this will, at some point in their life, have had the unpleasant experience of being stung by stinging nettles. But what chemicals do stinging nettles contain that elicit this effect? Further to that, a commonly espoused remedy for the stings, in the UK at least, is to rub dock leaves on them – but does this actually work, or is it just a widespread myth? This graphic sorts the nettle sting remedy fact from the fiction.

The nettle species, Urtica dioica, actually encompasses six different subspecies, all but one of which have stinging hairs. It is more common in areas with moist soil – which explains its ubiquity in the UK! It’s also abundant in Asia, North America, much of Europe, and even some Northern African countries. Despite it being so widespread, however, there’s still a lot we don’t know about stings from stinging nettles.

What we do know well is how they occur. Stinging nettles are covered with countless tiny hollow hairs called trichomes. When something brushes against these hairs, their very fragile silica tips break off, and the remainder of the hair can then act like a needle. It pierces the skin, and releases a cocktail of various chemicals from the base of the hair, and it’s these that cause the sting.

What’s in this mix of different chemicals that causes the sting? Whilst we still haven’t identified every single compound in the mixture, we have some idea. We used to think that the main component was formic acid, the same compound contained in ant venom. Whilst formic acid is certainly capable of causing a stinging sensation, and it is present in stinging nettles, it’s now thought that it’s present in too low a concentration to account for the extended pain of a stinging nettle sting.

Other chemicals contained in the stinging nettle venom, and the ones we now think are primarily responsible for the pain it induces, are histamine, acetylcholine, and serotonin. Serotonin, in particular, might sound familiar – it’s produced in our bodies, and sometimes referred to as the ‘happy hormone’, though it’s actually responsible for a number of other roles too. When injected by the stinging nettle, however, it functions as an irritant, leading to pain. Acetylcholine is another neurotransmitter that can accomplish a similar effect, and you might remember histamine from previous discussions of allergies, particularly hayfever. In the venom, histamine causes inflammation and pain.

Whilst all of the above contribute to the painful experience of a nettle sting, it’s still not the full story. Independently, they don’t explain why the effect of a nettle sting is so prolonged. It’s possible that there are synergistic effects between them and other chemicals in the venom mixture. Additionally, tartaric acid and oxalic acid are two compounds, isolated in a different species of stinging nettle, which were implicated in the drawn-out effect. However, further studies could reveal that there are still gaps in our current understanding.

So, you’re out and about, and get stung by a stinging nettle – what do you do? In some countries, your immediate reaction would probably be to hunt for a dock leaf to rub on the affected area. To many, this is almost second nature, but is it actually in any way effective? Anecdotally, it certainly seems to be, but actually there’s little in the way of scientific evidence.

Some have claimed that the dock leaf’s sap is alkaline, which neutralises the acidic compounds in the nettle sting. Even if this is the case, however, we’ve already pointed out that it’s not just the acidic compounds in nettle venom that are problematic. Additionally, dock leaf sap actually isn’t alkaline, so the whole argument falls apart. It’s certainly not a neutralisation reaction that’s soothing the sting.

Another suggestion is that dock leafs contain a natural antihistamine, which prevent histamine in the venom from producing inflammation and pain. This sounds like a decent theory – but there’s no scientific evidence that dock leaves do contain an antihistamine. After a lot of hunting, the only paper I could find naming a specific compound references another paper that supposedly shows dock leaf to have high levels of chlorphenamine. However, the paper being referenced doesn’t actually contain any mention of this at all. The ‘dock leaves contain antihistamines’ claim is a widespread one – a quick google will show as much – but in all cases it is unsubstantiated, and the trail of breadcrumbs always leads back to the same study which seems to be erroneously referencing a finding that doesn’t exist. As I result, I’m not convinced it isn’t just a total fabrication. (Of course, if anyone can provide evidence to the contrary, it’d be great to see!).

[Edit: 20/05/2016: Since writing this article, a paper which shows that dock leaf extract can have some effect on serotonin in the nettle sting has been brought to my attention. While it’s far from conclusive, being around 60 years old and only mentioned in conference proceedings, it does at least hint at the possibility of there being a chemical basis to dock leaves’ effects. Without any further or more recent research to back it up, the jury’s still out as to whether dock leaf acts merely as a placebo or not – more research needed!].

Another oft-suggested remedy is applying calamine lotion to the skin. Calamine is usually a mixture of zinc oxide and a small amount of iron (III) oxide, and is unsurprisingly the main ingredient in calamine lotion. It’s an anti-pruritic (anti-itching) agent, which is commonly applied to insect stings (which we’ve also looked at previously) to ease itching. Its efficacy has also been debated at times, and it won’t completely nullify the pain of the nettle sting, but it may help take the edge off.

Other, stranger methods of treating stings have also been suggested. Urinating on them is one that crops up more often than you might expect, but it’s likely to have little or no effect – and whilst we’re on the subject, there’s little point in urinating on a jellyfish sting either. Plantain leaves, much like dock leaves, are also a common remedy in some countries, though again, there’s currently no scientific evidence that they have any particular chemical effect.

One remedy that will alleviate the pain of the sting somewhat is the use of antihistamine or corticosteroid creams. Both of these prevent the action of histamine. Whilst, of course, it doesn’t do much to some of the other chemical components of the venom, preventing histamine’s action does at least help to reduce inflammation and some of the pain. Currently, this is the only remedy for nettle stings for which there is concrete scientific evidence.

So, next time you’re out walking and get stung by a nettle, there’s nothing wrong with hunting for the distraction or placebo effect of a dock leaf. But, even better, maybe have some antihistamine cream pre-packed as well!

Enjoyed this post & graphic? Consider supporting Compound Interest on Patreon, and get previews of upcoming posts & more!

DOWNLOAD

SUBSCRIBE

The graphic in this article is licensed under a  Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. See the site’s content usage guidelines.

References & Further Reading