The largest trial to date suggests that butterbur extract is as good as a standard antihistamine for treating hay fever.

Previous research has suggested that extract of the plant butterbur might be a useful treatment for allergic rhinitis, or hay fever. The remedy produces substances that can inhibit inflammation. The roots have been used for many years, but now it is the leaves that are under investigation. Researchers in Germany and Switzerland have subjected this herbal remedy to a thorough clinical trial.

They looked at 330 patients suffering from hay fever in 11 different centers. Treatment with butterbur extract proved as effective as that with a conventional antihistamine. In addition, butterbur did not lead to drowsiness. The results are promising, because they mean that butterbur can relieve the misery of allergic rhinitis in those who can't, or don't want to, take antihistamines.

Source
Phytotherapy Research online 22nd August 2005