High doses of oregano oil can build toxic levels of thymol in your liver, kidneys or nervous system. Because essential oils are so concentrated, many are toxic in small amounts if taken orally. Limit use to three daily doses for a maximum of three weeks.
In a 1991 study, the International Journal of Food Microbiology reported that oregano oil depletes levels of Lactobacillus plantarum and Leuconostoc mesenteroides, beneficial bacteria that live in the human colon to aid with digestion and protect you from bad bacteria. If you take oregano oil, you might be at risk for gastrointestinal illnesses unless you add a probiotic to your diet. Mayo Clinic nutritionist Katherine Zeratsky, R.D., L.D. recommends probiotic supplements with lactobacillus or other "live and active cultures.