Ask the Doctors by Eve Glazier, M.D. and Elizabeth Ko, M.D

Oregano's Effects on Humans Need to Be Studied More Extensively

Dear Doctor: What are the benefits of oregano oil?

Dear Reader: Oregano is well-known for its culinary versatility -- I like it with pasta and in chimichurri sauce -- but it has purported health benefits as well. A native of the Mediterranean region, the plant was used by the ancient Greek physician Hippocrates as an antiseptic and as an aid for digestive and respiratory ailments.

The beneficial effects of oregano appear to be from two specific chemical compounds, carvacrol and thymol. Both have been shown to have activity against multiple bacteria in laboratory settings. Among the many subspecies of oregano, all of them contain these two compounds -- known as phenols -- in some proportion. Oregano contains other phenol compounds as well, potentially boosting its antibacterial effects.

These compounds could be at least part of the reason why both the oregano plant and oregano oil inhibit the replication of intestinal pathogenic bacteria, such as Escherichia coli, Bacillus cereus and Salmonella, as well as the respiratory pathogens Strep. pyogenes, Staph. aureus and Moraxella. A 2014 study looked at the effect of adding either oil of the oregano plant (Origanum vulgare), carvacrol, thymol or a control to an antibiotic regimen against the organism Staph. aureus. The oregano oil and each of the phenols increased the inhibition of bacteria when added to the drug tetracycline, but none had any effect on the inhibition caused by the drugs erythromycin or norfloxacin. Also, oregano alone and carvacrol alone have shown activity against the norovirus, which causes gastrointestinal illnesses.

As for whether this anti-microbial effect translates to the world outside of a petri dish, only one good human study shows any efficacy. It looked at 14 patients who had non-pathogenic (not linked to illness) parasites in their stool. The participants were given 600 milligrams of oregano oil (Origanum vulgare) for six weeks. The study showed reduction or elimination of the parasites with the use of oregano oil. There was no placebo portion of the trial.

When it comes to cancer, Origanum vulgare has shown inhibition of cells in both breast and colon cancer. Another species, Origanum majorana, commonly known as sweet marjoram, stops the replication of liver cancer cells within the lab. A 2017 study in the European Journal of Nutrition looked at rats that were given medications to induce breast cancer. The 75 rats were separated into three groups: 25 were given 3 percent Origanum vulgare, 25 were given 0.3 percent and the last 25 were not given oregano. After 15 weeks of the study, the group given 0.3 percent oregano had a 44 percent reduction in the incidence of breast tumors. However, the rats given the 3 percent oregano had no reduction in the incidence of breast tumors. So maybe there is a Goldilocks zone for oregano. But again, human studies are nonexistent.

Some people report nausea and bloating with oregano oil, but overall, it appears safe. A 90-day study of oregano oil in rats found no adverse effects.

As for whether oregano oil has benefits, the answer is a solid maybe. No controlled human trials can even say whether it's effective against either infection or cancer. Some of the individual components such as carvacrol and thymol should be further studied in humans.

(Send your questions to, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)