Oil of oregano is a very valuable substance used in the practice of alternative medicine. It does, however, have some negative side effects. These effects are most often noted when the oil of oregano is used improperly.
Oil of oregano has been used in folk medicine both internally and externally. Only the wild variety grown in the Mediterranean is safe for internal use.
The ingredient in oil or oregano called thymol has the potential to be toxic at high doses. You should only use oil made from Origanum vulgare and start out with a very small dose until you see how your body is going to react.
Oil of oregano is a pathogen and causes organisms that don't belong in the body to die off. This can cause headache, nausea and dizziness if a large die-off happens at one time. Prevent this by drinking large amounts of water when you take oil of oregano.
The skin can be burnt by oil of oregano, especially when applied to sensitive areas such as mucous membranes and the genitals. Dilute the oil of oregano with coconut oil or olive oil before applying to the skin.
Never use oil of oregano without consulting your physician first if you are pregnant or nursing.