Two double-blind studies have compared butterbur extract to standard antihistamine drugs in people with hay fever. The first compared it with the drug cetirizine (Zyrtec) and found the drug and butterbur extract relieved symptoms equally well. However, cetirizine caused significantly more adverse effects, including a high rate of drowsiness.⁶ The second study compared butterbur extract with fexofenadine (Allegra) and placebo. Butterbur extract was as effective as fexofenadine at relieving symptoms, and both were significantly better than placebo.⁷

People with hay fever had better symptomatic relief and reductions in levels of immune cells associated with allergic reactions (eosinophils) when treated with an herbal formula containing horny goat weed compared with a formula without horny goat weed and another herb by itself.⁸ Traditionally 5 grams (1 tsp) of horny goat weed is taken three times per day, usually after being simmered (decocted) in 250 ml (1 pint) of water for 10 to 15 minutes.⁹

In a double-blind study, an extract of the butterbur plant (Petasites hybridus) was significantly more effective than a placebo at improving symptoms in people suffering from seasonal allergic rhinitis.¹⁰ The study used a preparation standardized to contain 8 mg of total petasin per tablet. One tablet was administered either two or three times a day for two weeks; the larger amount was found to be more effective than the smaller amount.

In a double-blind study, adult asthma patients taking inhaled steroids took either butterbur extract or placebo.¹¹ There was a significantly greater improvement in airflow in the group that took butterbur extract compared with those who took placebo. A study without a control group showed that people with mild asthma, most still taking various anti-asthma medications, had better airflow but actually showed some evidence of having more frequent asthma attacks when they took butterbur.¹² Therefore more rigorous studies are needed to know how effective butterbur is in people with asthma.

Double-blind trials have demonstrated that butterbur extract can reduce the frequency of migraine attacks significantly better than placebo.^13,14 One study has also shown it helps reduce the frequency of migraine headaches in children and adolescents, though there was no control group so it is not as clear just how effective butterbur extract is in this population.¹⁵ In a double-blind trial, supplementing with an extract of butterbur (Petasites hybridus) for four months was significantly more effective than a placebo at reducing the frequency of migraine attacks.¹⁶ The amount of butterbur found to be effective was 75 mg twice a day of an extract standardized to contain at least 15% petasins. A smaller amount (50 mg twice a day) was ineffective. The most common side effect was burping.