Which plant's stinging leaves are used to treat hay fever and joint pain?

The plant with stinging leaves traditionally used for hay fever and joint pain is stinging nettle (*Urtica dioica*).

Stinging nettle is a perennial herb whose leaves and stems carry hollow hairs called trichomes. When touched, the hair tips act like tiny needles and inject irritating chemicals, including histamine, acetylcholine, and serotonin. This produces the familiar burning sensation and explains why the plant is handled carefully despite its long history as food, fiber, tea, and medicine.

Traditional herbal practice has used nettle preparations for allergic complaints such as hay fever and for rheumatic or arthritic pain. A related practice, urtication, involves deliberately applying nettles to the skin to provoke warmth and inflammation. Nettle leaves are also eaten after cooking, which neutralizes the sting.

A common mix-up is dock, because broad-leaved dock often grows near nettles and folk tradition claims it soothes a sting. Dock is not the plant named in this question. Another useful distinction is that “stinging nettle” refers to the plant’s irritating hairs, not to an allergy caused by eating properly prepared leaves.