Hi, Definitely, oregano oil may have side effects, particularly, if it is taken exceeding the recommended dose. Mild side effects include stomach upsets like nausea, vomiting, hyperacidity and gastric distress etc. Other major side effects may be allergic symptoms like itching, rashes, sneezing, runny nose & even asthma as well as swelling of nasal passages. These allergic symptoms may be more frequent, if you are allergic to the plants of Lamiaceae family like basil, mint, lavender, sage or hyssop etc., as it also belongs to the same family. It can also hamper with the absorption of iron, copper & zinc in the body. Hence, it can cause easy fatigability, particularly in persons with low iron levels. Besides this, it can also cause a drop of blood sugar, particularly in patients already on sugar lowering medicines or herbs resulting in feeling of weakness. However, if the symptoms are mild, you can wait for some time, as these effects will go on its own within some time after stoppage of supplement. If allergic symptoms are present, then anti-allergic like cetirizine or loratadine etc. may be taken. Still, if the problem does not subside within some time or the symptoms of allergy or other side effects become more intense, then it will be always better to consult your doctor and treatment done accordingly. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Satnarain Bansal, General & Family Physician