There has been a great deal of controversy regarding the identity of nettle’s active constituents. Currently, it is thought that polysaccharides (complex sugars) and lectins are probably the active constituents. Test tube studies suggest the leaf has anti-inflammatory actions. This is thought to be caused by nettle preventing the body from making inflammatory chemicals known as prostaglandins.¹ Nettle’s root affects hormones and proteins that carry sex hormones (such as testosterone or oestrogen) in the human body. This may explain why it helps benign prostatic hyperplasia (BPH).² Although less frequently used alone like saw palmetto or pygeum, some limited clinical trials suggest benefit of nettle root extract for men with milder forms of BPH.³

Nettle leaf also contains a variety of flavonoids, which may have antihistamine effects. A preliminary trial reported that capsules made from freeze-dried leaves reduced sneezing and itching in people with hay fever.⁴ Further studies are needed to confirm this finding, however.

The historical practice of intentionally applying nettle topically with the intent of causing stings to relieve arthritis has been assessed by a questionnaire in modern times.⁵ The results found intentional nettle stings safe, except for a sometimes painful, sometimes numb rash that lasts 6–24 hours. Additional trials are required to determine if this practice is therapeutically effective.