Paradoxically, the less evidence that exists to support the use of of a treatment, the more passionate its supporters seem to be. I learned this early in my career as a pharmacist. One pharmacy I worked at did a steady business in essential oils. And king of the oils was oil of oregano. Not only were there several different brands of the basic oil, they were different forms, including capsules, creams and even nasal sprays. Not aware of any therapeutic benefits, I would ask customers what they were using it for. I rarely heard the same condition described: skin infections, athlete’s foot, head lice, colds, sore throats, “parasites”, “yeasts”, diabetes, allergies and ringworm were apparently no match against the judicious use of oregano oil. Intrigued, I took a closer look.

Long before our scientific understanding of bacteria and antimicrobials, infected wounds were packed with different products in an attempt to minimize the odour, and hopefully speed healing. It’s likely that someone happened upon a fragrant herb and discovered that it seemed to help treat wounds (or at least, cover some of the smell). Given there have been some amazing drugs with powerful effects that have emerged from natural products, it’s certainly plausible that oil of oregano could have biological and therapeutic effects. Oregano (Origanum vulgare) leaves contain a wide variety of chemical compounds, including leanolic acids, ursolic acids, and phenolic glycosides. Phenolic compounds make up to 71% of the oil. Carvacrol, thymol, cymene, and terpinine and are found in oregano leaves and do appear to have biological effects. It’s these chemicals that are proposed to be the parts with beneficial effects.

The claims made by one manufacturer are unambiguous:

Oreganol P73 is the most powerful germ killer with scientifically proven results against almost every virus, bacteria, parasite, and fungi. The complexity of the phytochemical matrix in Oreganol P73 possesses a broad spectrum of antimicrobial properties that are safe for prolonged use. The oil can be used topically and internally. Oreganol P73 is the medicine chest in a bottle, especially since it is proven never to harm the internal organs, even when used daily for health maintenance.

So if we accept the manufacturer’s claims at face value, there should be evidence demonstrating oregano oil is both safe and effective when used internally and externally. There is apparently also adequate long-term safety data to demonstrate that it can be safely used on a daily basis.

Effectiveness

When we contemplate administering a chemical to deliver a medicinal effect, we need to ask the following:

1. Is it absorbed into the body at all?
2. Does enough reach the right part of the body to have an effect?
3. Does it actually work for the condition?
4. Does it have any hazardous, unwanted effects?
5. Can it be safely eliminated from the body?

These questions are usually answered through a series of investigations, starting with preclinical (test-tube) studies, and moving into to animal trials, and then to human clinical trials that start with validating safety, and then progress to investigating efficacy and safety.

The short answer is that there’s little beyond animal studies to demonstrate that the ingredients in oregano oil have any effects. One of the best reviews seems to come from the McCormick Science Institute (yes, the spice company).  They commissioned a paper on oregano by Keith Singletary that appeared in the journal Nutrition Today in 2010. Happily, though the journal is paywalled, McCormick is hosting the paper on its own website.

1. Is oil of oregano absorbed? Some parts of the oregano do appear to be absorbed but the bioavailability of its different chemical constituents has not been verified. So we can’t be certain which components are reaching the circulation.

2. Does enough reach the right part of the body to have any beneficial effect? It’s not clear where the chemicals in oil of oregano act in the body, as no research has been done to show that it is adequately absorbed. However, there is some evidence to suggest that oregano may be implicated in inducing abortions in mice, so some parts of the herb must be absorbed, if this a causal effect. When applied to body surfaces or skin for topical effect, oil of oregano is more likely to reach high concentrations, at least locally, and then possibly deliver a medicinal effect. This makes topical effects seem much more plausible than ones that require ingestion.

3. Does it actually work for the condition? There is no published evidence to demonstrate that that oil of oregano is effective for any medical condition or illness. The McCormick review notes that that data for every condition evaluated is “preliminary, inconclusive.”  There is some very limited evidence to suggest that it might be useful for parasite infections — but given the evidence consists of only one study with 14 patients, and no placebo comparison, we really have no idea if the oregano oil itself was effective.

Let’s consider how oil of oregano might treat an infection. Bacteria are killed by antimicrobials based on a specific dose-response relationship. The minimum inhibitory concentration (MIC) must be reached at the site of an infection. Then there’s a concentration range where the bacteria (or viruses, or fungi, or parasites, depending on what you’re treating) are killed, typically in rough proportion to the dose. Keep increasing the dose, and the effect plateaus. If you can hit the MIC without causing side effects or toxicity, congratulations: you have a potential therapeutic agent.

There’s some evidence out there demonstrating that oil of oregano will kill different species of bacteria, etc in the test tube or Petri dish ( in vitro).  If I pour a pile of salt, lime juice, Cointreau, or tequila on a Petri dish, it will likely kill most bacteria too — but that doesn’t mean margaritas can treat pneumonia. It’s not difficult to kill bacteria if you change the conditions enough that it cannot live. So while it’s easy to get high concentrations of oregano in a test tube and subsequent positive effects, these effects are meaningless in the human body unless we can achieve similar concentrations, without any toxicity. And this has not been demonstrated with oil of oregano, or its individual chemical ingredients.

4. Does oil of oregano have any hazardous, unwanted effects? Natural does not mean safe. There are some reports of gastrointestinal upset with oil of oregano. There are also reports of allergic reactions. There is no evidence to suggest that oil of oregano, used at high (medicinal) amounts, may be used safely in pregnant or breast feeding women. However, when used in cooking, and as part of a regular diet, there is also no evidence that causes harm in pregnancy or breastfeeding. Animal studies show that if you give enough carvacrol, it will kill, though.

5. Can oil of oregano be safely eliminated from the body? So little published research exists on oil of oregano there is no way to determine if oregano oil is non-toxic. Certainly, at low doses, when used as a food, there is no reason to have any concerns. But at higher doses, and particularly with regular use, there is no data to sugges it’s safe to consume all that carvacrol, thymol, cymene, and terpinine. As we have no idea if and how oregano oil works, we have no information to estimate what a proper dose might be. Doses published by manufacturers are not based on any published evidence.

Conclusion

Oil of oregano, and the claims attached to it, is a great example of how interesting laboratory findings can be wildly exaggerated to imply meaningful effects in humans. A few small studies have been conducted, mainly in the lab, and advocates argue this is evidence of effectiveness. The rest is all anecdotes.

Despite the hype, there is no persuasive evidence to demonstrate that oil of oregano does anything useful in or on our bodies. And while it is popular, there is no science to support the use of oil of oregano for any medical condition. Suggesting that this herb is can effectively treat serious medical conditions like diabetes, asthma, and cancer is foolish and dangerous. If you’re ill, stick to the proven science, and save your oregano for cooking.

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