It is considered a mild remedy with minimal toxicity. An allergic reaction is possible and side affects of this could include difficulty breathing, speaking, or swallowing, facial swelling, and itching. Do not use if you have had an allergic reaction to it or to other herbs of the same family such as thyme, hyssop, basil, marjoram, mint and sage. ¹⁸Do not take oregano within 2 hours before or after taking an iron supplement or eat with high iron foods. Oregano is a uterine stimulant and is therefore contraindicated during pregnancy. Essential oil of oregano is a volatile oil and should never be used directly on the skin without diluting first in a carrier oil such as olive oil.