The common remedy of rubbing dock leaves on nettle stings probably does not work in any way unique to the plant, according to John Tregoning, a professor at Imperial College London.
When stinging nettles brush against skin, tiny hairs release chemicals including histamine, acetylcholine and serotonin that cause burning, itching and redness. Dock leaves contain little evidence of substances capable of significantly relieving this reaction, Tregoning says.
The perceived benefit likely comes from other factors. Rubbing any leaf against the skin creates a competing sensation that temporarily distracts from pain by overstimulating nerve cells, he explains. The time spent searching for dock leaves may also relieve the short burst of pain through distraction. A cooling or soothing effect from the sap is possible, but "none of these things are unique to dock leaves," Tregoning says.
A small study comparing dock leaves with lettuce leaves as a treatment in nine medical students found no meaningful difference between the two. Tregoning suspects any perceived benefit is most likely a placebo effect.