Avoid in individuals with a known allergy or hypersensitivity to oregano. Possible cross-sensitivity with other herbs from the Lamiaceae family including hyssop (Hyssopus officinalis), basil (Ocicum basilicum), marjoram (Origanum majorana), mint (Mentha piperita), sage (Salvia officinalis), and lavender (Lavendula officinalis).

Itching and swelling of the lips and tongue, difficulty speaking and breathing, and face swelling have been reported following the ingestion of pizza containing oregano.

Based on historical use, it appears that oregano is well tolerated in recommended doses. However, there are no available reliable clinical trials demonstrating safety or efficacy of a particular dose or for a recommended treatment duration.

Oregano may lower blood sugar levels. Caution is advised in patients with diabetes or hypoglycemia, and in those taking drugs, herbs, or supplements that affect blood sugar. Serum glucose levels may need to be monitored by a healthcare provider, and medication adjustments may be necessary.

Oregano is not recommended at doses above those normally found in food due to a lack of available scientific evidence. An over-the-counter product containing oregano as one of four herbal ingredients (Carachipita® - pennyroyal (Mentha pulegium), yerba de la perdiz (Magiricarpus pinnaus) and guaycuru (Statice brasiliensis)) has been linked with case reports of induced abortion.

There is no proven effective dose for oregano. Oregano has been taken in doses of 200 milligrams of emulsified oil three times daily with meals for six weeks for the treatment of enteric parasites. As a dietary supplement, two capsules (dose unknown) once or twice daily has been recommended with meals, or a few drops of oil of oregano can be added to milk or juice.

Oregano oil has also been applied topically (on the skin), and shampoos and teas (gargle, mouthwash) are commercially available. For use as a bath additive, 100 grams of dried oregano leaf may be steeped in one liter of water for 10 minutes, strained and added to a full bath.

There is no proven effective dose for oregano in children.

Oregano may lower blood sugar levels. Caution is advised when using medications that may also lower blood sugar. Patients taking drugs for diabetes by mouth or insulin should be monitored closely by a qualified healthcare professional, including a pharmacist. Medication adjustments may be necessary.

Oregano may have phytoestrogenic effects. Interactions with hormonal agents are theoretically possible.

Oregano may have anti-thrombin effects. Interactions with anticoagulants are theoretically possible.

Oregano may lower blood sugar levels. Caution is advised when using herbs or supplements that may also lower blood sugar. Blood glucose levels may require monitoring, and doses may need adjustment.

Because oregano contains estrogen like chemicals, the effects of other agents believed to have estrogen-like properties may be altered

Oregano may have anti-thrombin effects. Interactions with anticoagulants are theoretically possible.