American Wormseed (Dysphania ambrosioides, formerly Chenopodium ambrosioides), also known as epazote, has a long history of traditional use as an anthelmintic—meaning it is used to expel parasitic worms and other internal parasites. Its use in treating intestinal parasites, including Giardia lamblia, is documented in folk medicine across the Americas and parts of Asia. Historical ethnobotanical records indicate that indigenous peoples and traditional healers have employed the seeds and leaves of this plant for gastrointestinal complaints believed to be due to parasite infection.

The primary active compounds, such as ascaridole, have demonstrated some antiparasitic activity in vitro and in animal studies, but there is limited direct scientific evidence specifically targeting Giardia in humans. The plant’s essential oil was historically used as an ingredient in commercial anthelmintic preparations, but concerns about toxicity (especially ascaridole's potential for severe side effects) have led to reduced use in modern medicine. A few laboratory studies suggest potential activity against protozoan parasites, though robust clinical trials in humans for giardiasis are lacking.

In summary, the use of American Wormseed for giardiasis is primarily justified by traditional practice and historical precedent, with only modest supporting evidence from scientific research. Due to toxicity risks, it is not recommended as a first-line treatment in current clinical practice.