Oregano

Oregano is an herb that is alleged to have antimicrobial effects and has been studied as an herbal antibiotic for use in treatment of Small Intestine Bacterial Overgrowth (SIBO).

Forms
Oregano is available as "Oil of Oregano", an essential oil which contains concentrated chemical compounds extracted from the leaves of the oregano plant. The oil has a strong scent and flavor that is very similar to the common oregano spice, but much stronger. A drop of the oil on skin or in the mouth produces a warming or burning sensation. Instructions for dosing typically recommend diluting the oil with an inert carrier oil, such as Olive Oil, to reduce the sensation. Dosage recommendations vary by source, but are typically a few drops administered two or more times per day.

Oregano is also available as capsules containing dried oregano leaves.

Evidence
A retrospective chart review published in 2014 compared an herbal therapy protocol (which included Oregano) to therapy with the synthetic antibiotic Rifaximin. Efficacy of the herbal therapy, as measured by Lactulose breath test conducted post-treatment, was superior. The study included patients newly diagnosed with SIBO, and patients in each group were treated for 4 weeks. The study had several important caveats:
 * The herbal protocol included a mix of herbs, not oregano alone.
 * The Rifaximin group was administered only 1200 mg per day, whereas the dosage in for SIBO treatment is typically 1650 mg / day.
 * It was not a prospective or randomly controlled trial.

United States
In the United States, both Oregano capsules and Oil of Oregano are available without a prescription. The cost of a bottle of Oil of Oregano is in the low tens of dollars and contains a 2-4 week supply, assuming typical dosing.