Oregano (Origanum vulgare) has been investigated for its antimicrobial properties, including activity against the protozoan parasite Giardia lamblia (the cause of giardiasis). The primary scientific interest centers around oregano essential oil and its main active phenolic compounds, carvacrol and thymol. Several in vitro studies have shown that oregano oil and these constituents exhibit inhibitory effects on the growth and viability of Giardia trophozoites. For example, research published in Phytotherapy Research (2000) demonstrated that oregano oil could reduce Giardia infection in gerbil models and inhibit trophozoite growth in laboratory cultures.

However, human clinical evidence is limited. A small pilot study (Force et al., 2000) reported the use of oregano oil in humans with intestinal parasites, including Giardia, with some participants showing improvement. Still, the trials were small, lacked rigorous controls, and the results have not been widely replicated. There is also limited information on optimal dosing and safety for this specific use.

Summary: Laboratory and animal studies provide preliminary scientific evidence that oregano oil and its components may have anti-Giardia activity, but robust human data are lacking. Thus, the evidence level is rated as 2/5—suggestive but not definitive. Oregano has not been a major part of traditional medicine for giardiasis specifically, and its modern use is mainly based on emerging research rather than a long-standing tradition.