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This dried herb contains a few constituents, including volatile oil (up to 3%), such as carvacrol, thymol, and borneol, plus flavonoids, rosmarinic acid, triterpenoids (e.g. ursolic and oleanolic acid), sterols, and vitamin A and vitamin C.⁴ The thymol and carvacrol contents in oregano are responsible for its antimicrobial and antifungal effects.⁵ A test tube study demonstrated that oil of oregano, and carvacrol in particular, inhibited the growth of Candida albicans far more effectively than a commonly employed antifungal agent called calcium magnesium caprylate.⁶ Clinical studies are still needed to confirm these actions in humans.

In addition to its anti-fungal action, and according to the results of another test tube study from Australia, oregano oil has a strong anti-microbial action against a wide number of bacteria, including Escherichia coli, Klebsiella pneumoniae, Salmonella enterica, and Staphylococcus aureus.⁷ Other test tube studies have shown that oregano from the Mexican (Lippia) species was more effective than the prescription medication tinidazol in inhibiting the parasite giardia (Giardia duodenalis).⁸ In another test tube study, volatile oils of oregano, thyme, cinnamon, and cumin were individually able to stop the growth of another food-borne pathogen called Aspergillus parasiticus. Higher concentrations of these volatile oils were also able to stop the production of aflatoxin, a potent poison from the food mouldAspergillus.⁹ Together these facts suggest the volatile oils in oregano used during food processing have an important role in preventing the spoilage of food and in reducing the risk of ingesting harmful bacteria, fungi, and parasites. Again, these actions have not yet been confirmed by human clinical trials.

The German Commission E does not approve oregano for any medical indication.¹⁰