Barberry (Berberis vulgaris) has been used traditionally in herbal medicine to treat gastrointestinal infections, including those caused by parasites such as Giardia lamblia. The primary active constituent in barberry is berberine, an isoquinoline alkaloid that has demonstrated antimicrobial and antiprotozoal effects in laboratory studies. Several in vitro studies have shown that berberine can inhibit the growth of Giardia lamblia trophozoites, supporting the rationale for its use against giardiasis. In addition, limited animal studies have suggested that berberine and barberry extracts may reduce giardial infection and ameliorate intestinal symptoms in infected animals.

Human clinical data, however, are limited. A few small trials have indicated that berberine may be as effective as metronidazole (the standard pharmaceutical treatment) in clearing Giardia infections, though the number of participants and methodological quality of these studies are generally low. Traditional use in various systems of medicine (e.g., Ayurveda, Traditional Chinese Medicine) also supports the use of barberry and related berberine-containing plants for digestive infections. However, due to the paucity of high-quality human trials, the evidence is best characterized as preliminary but promising.

In summary, there is a scientific basis for the use of barberry in giardiasis, primarily due to pharmacological studies of berberine, but robust clinical evidence in humans is lacking, supporting an evidence rating of 2 out of 5.