Oregano oil

What it is

Oregano is a flowering herb of the mint family (Lamiaceae) whose dried leaf is a kitchen spice and whose leaves and flowering tops are steam-distilled or solvent-extracted to make oregano oil. The two monoterpene phenols carvacrol and thymol dominate the oil and carry most of its measured antioxidant and antimicrobial activity; the leaf also contains other polyphenols. Products sold as "oregano oil" range from a concentrated essential oil to a 500 mg dried-herb capsule, and dried oregano itself is one of the most commonly adulterated spices on the market.

What the research says

The honest summary is that oregano is studied far more in test tubes than in people. Laboratory work on carvacrol and thymol is extensive — a systematic review of 77 studies found 69 in cells, 10 in animals and not one human trial — but the human record is two small, short randomised trials and no meta-analysis of clinical outcomes that we could find. One 24-soldier trial reported lower muscle-damage and oxidative-stress markers over two hours after a single 500 mg capsule, and one 75-patient nasal-spray trial in chronic rhinosinusitis reported very large symptom-score improvements; both are small, single-centre and unreplicated, and in both cases the published effect sizes are too large to be taken at face value. Against that sit a documented anticoagulant interaction and widespread commercial adulteration.

Evidence grade: Limited evidence.

What goes wrong

A published case report describes a 77-year-old woman stable on acenocoumarol whose INR rose to 6.42 within a week of starting a daily oregano infusion and normalised when she stopped it. (Source 1)

• Case report, Very low certainty.
• Size: 1 patient.
• Who: 77-year-old woman with atrial fibrillation, on acenocoumarol for 8 years, INR stable in range 2-3.
• How long: One 200 ml cup of oregano infusion daily for one week; followed to April 2024.
• Result: INR 6.42 and prothrombin time 17.9% on 6 September 2023, falling to 3.80 the next day after stopping, then 2.5 on day 4 of restarting the anticoagulant. The authors attribute it to CYP2C9/CYP3A4 inhibition plus carvacrol and thymol's own anticoagulant activity. A single unblinded case with a co-ingested herb (verbena), so causation is not established.
• Funding: not stated.

Limit of this finding: A single case report. The patient was also drinking verbena, so the effect cannot be pinned on oregano alone, and the CYP2C9/CYP3A4 mechanism and the anticoagulant activity of carvacrol and thymol are the authors' proposed explanation, not anything measured in this patient. It shows that this has been reported once, not how often it happens or that oregano tea causes it.

On September 6, 2023, during a routine check-up, the patient's INR was 6.42, and the PT was 17.9%.

A 2024 laboratory survey found 2 of 15 retail dried oregano samples adulterated with olive leaves, and cites an earlier 2001-2007 survey in which most samples carried undeclared or extraneous material, so a packet's actual oregano content can be well below what the label implies. (Source 2)

• Survey study, Low certainty.
• Size: 15 commercial oregano samples analysed in this paper; the 59% and 48% figures come from an earlier study it cites.
• Who: Retail dried oregano samples.
• How long: Not applicable - single-timepoint laboratory analysis.
• Result: This paper's own result: 2 of 15 commercial samples (13%) classified as adulterated, containing an estimated 31% and 43% olive leaves. Separately, the paper quotes a study carried out between 2001 and 2007 in which 59% of oregano samples contained extraneous material above 20% by weight and 48% contained undeclared material, usually olive leaves.
• Funding: not stated.

Limit of this finding: Two sets of numbers are in play and they come from different studies. This 2024 paper's own finding is 2 adulterated samples out of 15, and the 31% and 43% figures describe how much olive leaf was in those two individual packets — they are not the share of the market that is adulterated. The 59% and 48% figures are quoted by this paper from a survey carried out between 2001 and 2007; they are not a 2024 measurement and should never be reported as one.

Finally, the proposed methodology was applied to 15 commercial oregano samples, resulting in two of them being classified as adulterated with 31 and 43% of olive leaves, respectively.

What the evidence supports

In a small randomised trial in soldiers, a single 500 mg oregano capsule taken after a combat-readiness test was followed by different time-by-group changes in muscle damage and oxidative stress markers over the following two hours; the effect sizes the paper prints are not credible. (Source 3)

• Randomized trial, Low certainty.
• Size: 24 healthy male soldiers (12 oregano, 12 placebo)
• Who: Healthy male soldiers, age 24 +/- 3 years.
• How long: Single dose; blood sampled before, immediately after exercise, and 60 and 120 min after ingestion.
• Result: Time x group interactions for CK (p < 0.0001, d = 3.64), LDH (p < 0.0001, d = 1.64), MDA (p < 0.0001, d = 9.94), SOD (p < 0.0001, d = 1.88), TAC (p < 0.0001, d = 5.68), GPX (p < 0.0001, d = 2.38). The authors name the small sample and the single time-window as limitations.
• Funding: independent - the paper states it received no external funding and the authors declare no conflict of interest.

Limit of this finding: The effect sizes as printed cannot be right. A Cohen's d of 9.94 for MDA and 5.68 for TAC in 12 men per group would mean the two groups barely overlapped at all, which does not happen with blood markers; numbers that large normally signal a calculation or reporting problem, so treat them as uninterpretable rather than as the strength of the finding. The trial also gave one 500 mg capsule and followed blood for only 120 minutes, and the authors say they did not look at 24, 48 or 72 hours, so nothing here shows faster recovery from muscle damage despite what the paper's title says. The significant results are time-by-group interactions: the differences between the groups before the test and immediately after it were not significant.

The time × group interactions were found for CK (p < 0.0001, d = 3.64), LDH (p < 0.0001, d = 1.64), MDA (p < 0.0001, d = 9.94), SOD (p < 0.0001, d = 1.88), TAC (p < 0.0001, d = 5.68) and GPX (p < 0.0001, d = 2.38).

A single 75-patient trial at one Iranian clinic reported much larger symptom-score reductions with an oregano oil nasal spray than with fluticasone or sesame oil, but the figures it prints are internally inconsistent and implausibly large. (Source 4)

• Randomized trial, Low certainty.
• Size: 75 adults with chronic rhinosinusitis.
• Who: Adults with chronic rhinosinusitis without nasal polyps attending one otolaryngology clinic in Iran.
• How long: 4 weeks.
• Result: The paper reports reductions in mean change in SNOT-22 of 51.52 (95% CI, -55.79 to -47.24) for oregano, 21.60 (95% CI, -25.48 to -17.71) for fluticasone and 11.84 (95% CI, -13.18 to -10.51) for placebo, and mean differences of 29.92 and 39.68 points in favour of oregano. As printed, not one of those point estimates falls inside its own stated interval. Single-centre and unreplicated; the authors say generalisation should be explored further.
• Funding: not stated.

Limit of this finding: These numbers should not be relied on. Every confidence interval in the trial contradicts its own point estimate: a reduction of 51.52 points is paired with an interval running from -55.79 to -47.24, and the same mismatch runs through all five figures, so as published none of the estimates sits inside its own interval. A 51.52-point fall is also far outside anything else in the rhinosinusitis literature — the SNOT-22 scale runs from 0 to 110 and a change of roughly 8.9 points is the smallest one patients notice. This is one unreplicated trial of 75 people at a single clinic, and genuinely blinding a strongly aromatic oil against a steroid spray is very hard. Read it as a claim waiting to be replicated, not as evidence that oregano oil outperforms fluticasone.

The reduction of mean change in SNOT-22 scores were 51.52 (95% CI, -55.79 to -47.24), 21.60 (95% CI, -25.48 to -17.71) and 11.84 (95% CI, -13.18 to -10.51) points for those in the oregano, fluticasone and placebo group, respectively.

What the evidence does not support

A systematic review of the antitumour effects of oregano's two main constituents found no human studies at all - all 77 included studies were laboratory or animal experiments. (Source 5)

• Systematic review, Very low certainty.
• Size: 77 studies (69 in vitro, 10 in vivo) screened from 1,170 records.
• Who: Cell lines and animals; no human participants.
• How long: Studies published 2003-2021.
• Result: No clinical trial met inclusion. The review states that a standard and safe dose has still to be defined and that toxic or side effects remain to be determined; risk-of-bias judgement was largely impossible because studies did not report randomisation, allocation concealment or blinding.
• Funding: not stated.

Limit of this finding: Every one of the 77 studies in this review was preclinical: 69 in cells and 10 in animals, with no human trial meeting the inclusion criteria. Statements such as 'carvacrol appears to be more potent than thymol' describe cell cultures. Nothing here tells you what oregano, or its constituents, does to a tumour in a person, and the review's comment about poor reporting of randomisation and blinding is a judgement about animal experiments, not about clinical trials.

A total of 1,170 records were identified, with 77 meeting the established criteria. The studies were published between 2003 and 2021, with 69 being in vitro and 10 in vivo.

Where the evidence is mixed

In a retrospective chart review of newly diagnosed small intestinal bacterial overgrowth, a four-week herbal antimicrobial regimen was not shown to differ from rifaximin, but patients were not randomised and the report does not name the herbs used, so nothing here can be attributed to oregano oil. (Source 6)

• Cohort study, Very low certainty.
• Size: 104 patients (37 herbal, 67 rifaximin)
• Who: Adults with a positive lactulose breath test for SIBO at a US tertiary centre.
• How long: 4 weeks.
• Result: Negative follow-up breath test in 17/37 (46%) herbal vs 23/67 (34%) rifaximin, P=.24 — a non-significant difference, not a demonstration of equivalence. Patients were offered one treatment or the other rather than randomly assigned. Adverse effects reported in the rifaximin arm included one anaphylaxis, two hives, two diarrhoea and one Clostridium difficile, against one diarrhoea on herbs, a difference that did not reach significance (P=.22).
• Funding: not stated.

Limit of this finding: Three things here are easy to misread, starting with the paper's own title. 'Herbal Therapy is Equivalent to Rifaximin' rests on P=.24, which only means the gap between 46% and 34% could be chance in a group this size; with 37 and 67 patients the study had no power to establish equivalence, and it set no equivalence margin in advance. Second, patients were 'offered' one treatment or the other rather than randomly assigned, so the two groups may have differed from the start in ways nobody measured. Third, the herbal arm used a herbal regimen the report does not itemise, not oregano oil on its own, so the result cannot be credited to oregano. The safety contrast also did not reach significance (P=.22), and the two arms differed in size by nearly twofold, so the raw counts are not comparable as printed.

Of the 37 patients who received herbal therapy, 17 (46%) had a negative follow-up LBT compared to 23/67 (34%) of rifaximin users (P=.24).

Where the research disagrees

Whether oregano is a benign herb or a substance with meaningful drug-interaction risk

• Boukhalfa and colleagues, reporting the acenocoumarol case, case-report: Oregano is an endemic plant widely used in Algeria, whose anti-inflammatory properties are well established. However, its side effects, toxicity, and risk of interaction when combined with medications are less known. (Source 1)
• Costa and colleagues, systematic review of carvacrol and thymol, systematic-review: However, further in vivo studies with robust methodology are required to define a standard and safe dose, determine their toxic or side effects, and clarify its exact mechanisms of action. (Source 5)

How much

• Reference intake: No reference intake (RDA or AI) exists for oregano or oregano oil. It is a culinary herb and a supplement ingredient, not an essential nutrient, and no body we could reach has set an intake figure for it. (Source 3)
• Upper limit: No human upper limit or acceptable daily intake was found in the sources we could reach. A 2021 systematic review of carvacrol and thymol states that a standard and safe dose still has to be defined. EFSA opinions we located on Origanum vulgare essential oil concern feed additives for animals, and the EFSA and EMA pages returned HTTP 403 to our fetches. (Source 5)
• Studied: A single capsule containing 500 mg Origanum vulgare, taken once immediately after exercise (24 soldiers). (Source 3)
• Studied: An oregano oil nasal spray used for 4 weeks alongside lifestyle instructions (75 adults with chronic rhinosinusitis). (Source 4)
• Studied: A daily 200 ml cup of oregano (Origanum vulgare) infusion for one week - the exposure in the reported bleeding-risk case. (Source 1)

A common belief, and what the research shows

The belief: Oregano oil is a proven natural antibiotic that can be taken freely because it is just a kitchen herb.

What the research shows: The antimicrobial and antitumour work behind that reputation is overwhelmingly laboratory work: a 2021 systematic review screened 1,170 records and reports of its 77 eligible studies that "The studies were published between 2003 and 2021, with 69 being in vitro and 10 in vivo." No human trials met inclusion. Meanwhile a 2025 case report records an INR of 6.42 in a woman on acenocoumarol after a week of oregano tea: "On September 6, 2023, during a routine check-up, the patient's INR was 6.42, and the PT was 17.9%." A kitchen herb in a concentrated form is not automatically inert.

Questions and answers

What is it?

Oregano is a herb of the mint family. Oregano oil is the concentrated extract or essential oil of its leaves and flowering tops. Its two most-studied constituents are carvacrol and thymol, monoterpene phenols that account for most of its measured antioxidant and antimicrobial activity. (Source 3)

What does it do in the body?

In laboratory work oregano extract behaves as an antioxidant and antimicrobial, and those properties are the proposed route to its anti-inflammatory effects. What that translates to in people is barely mapped: the human evidence is two small short trials, not a body of literature. (Source 3)

Is it good or bad for you?

Culinary amounts of oregano are eaten worldwide without reported problems. Concentrated oil and strong infusions are a different matter: the anti-inflammatory reputation is much better established than the safety and interaction data, and at least one case report links oregano tea to over-anticoagulation. (Source 1)

How do you get more of it?

In the trials we found, people took either a 500 mg dried-herb capsule or an oregano oil nasal spray; traditionally it is drunk as an infusion of the dried leaf. This is a description of what studies gave participants, not a recommendation. (Source 3)

If it is harmful, what reduces it?

Oregano is not stored in the body and its reported effects reverse on stopping. In the published anticoagulation case the patient's INR came back down once the oregano and verbena infusions were discontinued, and stabilised when intake was limited and diluted. (Source 1)

Why might someone be low in it or missing it?

Does not apply in the nutritional sense: oregano is not an essential nutrient and there is no deficiency state. What does vary is how much genuine oregano a product actually contains, because the spice is commonly diluted with cheaper leaf material for profit. (Source 2)

Which whole foods contain it or feed it?

The plant itself is the food source: dried oregano leaf used as a spice, and oregano herbal tea, which is the traditional preparation described in the clinical literature. Concentrated oil capsules and sprays are supplement forms, not foods. (Source 1)

What happens if you do not have it?

Nothing is described in the literature. Oregano is not essential to human physiology and no deficiency syndrome has been reported; the reviews that exist study what added carvacrol and thymol do, not what happens without them. (Source 5)

We searched: Searched for oregano or Origanum vulgare deficiency, carvacrol deficiency and essential-nutrient status via WebSearch; the only systematic review located (77 studies of carvacrol and thymol, Frontiers in Pharmacology 2021) addresses added compounds in vitro and in vivo, not any absence state.

How can you test for it?

There is no validated clinical test of a person's oregano status, and none is described in the literature we searched. The validated tests that do exist are analytical tests of the product itself - for example gas chromatography-ion mobility spectrometry used to detect whether dried oregano has been cut with olive leaves. (Source 2)

References

1. Annales de Cardiologie et d'Angéiologie. Origanum vulgare et risque hémorragique, à propos d'un cas (Origanum vulgare and hemorrhagic risk, about a case). 2025. PMID 39793412, DOI 10.1016/j.ancard.2024.101856. Read the source
2. Foods. Detection of Adulterated Oregano Samples Using Untargeted Headspace–Gas Chromatography–Ion Mobility Spectrometry Analysis. 2024. DOI 10.3390/foods13040516. Read the source
3. Nutrients. Oregano (Origanum vulgare) Consumption Reduces Oxidative Stress and Markers of Muscle Damage after Combat Readiness Tests in Soldiers. 2023. PMID 36615794, DOI 10.3390/nu15010137. Read the source
4. Indian Journal of Traditional Knowledge, Vol 19(2), pp 341-349. Effect of oregano oil (Origanum Vulgare L.) on chronic rhinosinusitis: A randomized, double-blind, clinical trial. 2020. Read the source
5. Frontiers in Pharmacology. Antitumor Effects of Carvacrol and Thymol: A Systematic Review. 2021. DOI 10.3389/fphar.2021.702487. Read the source
6. Global Advances in Health and Medicine. Herbal Therapy is Equivalent to Rifaximin for the Treatment of Small Intestinal Bacterial Overgrowth. 2014. DOI 10.7453/gahmj.2014.019. Read the source