Stinging nettle (Urtica dioica), close-up. The stinging nettle is a herbaceous perennial flowering plant. It has hollow stinging hairs on its leaves and stems. When brushed against they inject chemicals into animal and human skin, creating a stinging sensation and a rash. The chemicals injected are acetylcholine, histamine, serotonin, moroidin, leukotrienes and sometimes formic acid. As the hairs grow upward along the stalk and outwards on the leaves, it is possible to touch them in the opposite direction without getting stung.