Oregano and Oregano Oil: Important Properties and Possible Side effects

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Oregano is a basic spice in many cuisines around the world, especially in the Mediterranean. It gives a spicy aroma and flavor to the dishes while it can be easily integrated into a wide variety of recipes, either as fresh, dried or even as an oil.

The plant is cultivated not only for use as a spice but also for receiving essential oil (oregano oil) from its leaves. Oregano oil is a liquid with a yellowish to reddish color and a thyme aroma. It is obtained by distillation in a very concentrated form and is considered to have important biological and medicinal properties.

Also, traditionally, people make oregano beverages and drink it to soothe various health problems. It is used as a natural expectorant for coughs, colds, sore throats, nausea, stomach upsets, phlegm, constipation, flatulence, bloating, diarrhea and irritable bowel syndrome.

The name of the plant comes from the words “mountain” and “ganos” (brilliance) which means that the mountain shines. There are at least 17 species of herbs in Greece that have the characteristic smell of oregano and are classified in the same genus (Origanum). The species that is most interested is the Origanum vulgare (Origanon the common) because it is more common in our country. It is considered to be the best quality oregano in the world and is referred to as “Greek oregano”.

Ingredients and nutritional value

Oregano has a high content of two aromatic substances, carvacrol and thymol. These are two antioxidants that belong to phenols. Carvacrol is a potent antibacterial and antifungal agent (1) while thymol is an isomer of carbacrol that supports the immune system (2). More than 60 chemical compounds have been detected in the essential oil of oregano, but carvacrol and thymol together account for 78-82% of all these substances. Greek oregano oil contains significant amounts of carvacrol which can reach up to 79%, while thymol can be up to 6%. In essential oils of other varieties it has been found that the percentages of carvacrol and thymol are approximately equal. Other ingredients are γ-terpene and r-cumin which are usually 5% and 7% respectively.

The nutritional value of oregano is important, although as a spice it is added in small amounts in the diet. For example, a teaspoon of dried oregano can give you about 8% of your daily vitamin K needs (3). In combination with other foods such as fruits and vegetables, oregano can provide a rich dose of antioxidants and can help you improve your health. Dried oregano has about 265 calories per 100 grams of weight. The same amount contains 1,701 International Units of Vitamin A, 2.3 mg of Vitamin C, 18 mg of Vitamin E, 622 mcg of Vitamin K, 177 mcg of thiamine, 528 mcg of riboflavin and 4.6 mg of niacin.

Here are four important properties of oregano, which can be extremely useful. It should be noted, however, that the studies were performed on a test tube or on experimental animals using a significant amount of the herb. Studies that use standard doses in humans are necessary to determine the benefit.

1.Oregano and microbes

Oregano oil is the oldest natural antiseptic with strong antimicrobial action. It can kill or stop the growth of bacteria and fungi. For example, a study in a test tube showed that oregano oil prevented the growth of Escherichia coli (coliform bacteria) and Pseudomonas aeruginosa (pseudomonas aeruginosa), two germs that can cause infection (4). Another study found that oregano was effective against 23 species of bacteria (5). Also, a test tube study compared the antimicrobial activity of the essential oils of oregano, sage and thyme and found that the most effective was oregano with second the thyme (6). In 2019, a laboratory study (7) found that carvacrol and thymol prevented the appearance of S. aureus in meat and dairy products.

 It seems that both carvacrol and thymol fight fungi depending on the amount used. The antifungal action of the other ingredients of oregano, such as γ-terpene and r-cement, is also known, but it is unknown whether there is a synergy of all these ingredients. It has been suggested that the antibacterial action of oregano is due to the ability of phenolic substances to penetrate the bacterial cell membrane and penetrate their cells, where they adversely affect the biochemical mechanisms of their metabolism.

2.Oregano and viruses

In addition to fighting bacteria, studies in test tubes have found that oregano and its ingredients can also protect against certain viruses. Carvacrol and thymol have been associated with antiviral properties. One study showed inactivation within one hour of the norovirus that can cause diarrhea, nausea and stomach pain (8). Another study found that carvacrol and thymol inactivated 90% of the herpes simplex virus within an hour (9).

3.Oregano and inflammation

Inflammation is a normal immune response that occurs as a result of illness or injury. However, chronic inflammation in the body contributes to the development of diseases such as heart disease, diabetes and autoimmune diseases. Carvacrol has been shown to have anti-inflammatory properties. In an animal study, it reduced the swelling in the feet of mice by up to 57% (10). Another animal study showed that a mixture of oregano and thyme essential oils reduced the number of inflammatory markers in mice with colitis or with inflammation of the colon (11). These studies used very concentrated amounts of the herb and studies with a normal dosage that could stop inflammation in humans are needed.

4.Oregano and Diabetes

Oregano may help manage type 2 diabetes repairing liver and kidney damage (12). They have noticed that some people already use oregano leaves and oil to manage high blood sugar levels.

Another study found that oregano extract improved type 1 diabetes in mice (13). The researchers said that this could be due to the antioxidant properties of the herb and its effect on the immune system.

Possible side effects and contraindications

Oregano oil is also available in capsule form. It is used for skin diseases such as e.g. acne and sores, after dilution, while taken orally for intestinal parasites. The beverage can be used for gargling before swallowing.

Can oregano have side effects?

The herb has a long history of safe use in food and food products. Most people will not experience side effects however, if you drink a lot of oregano beverage, more than four cups a day, you may feel some stomach upset.

In rare cases, an allergic reaction to oregano may occur. If you are allergic to mint, you may also be allergic to oregano. There are contraindications for people with bleeding disorders as oregano can increase the risk of bleeding. People who are going to have surgery should stop consuming it two weeks in advance.

There is also concern that during pregnancy, oregano, in large quantities, could cause miscarriage. Its safety is not known when used in medicinal quantities during breastfeeding. The use of oregano essential oil should be avoided by children under 4 years of age, which is true for most essential oils.


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  1. Antimicrobial activity of carvacrol: current progress and future prospectives.
  2. Antifungal activity and mode of action of thymol and its synergism with nystatin against Candida species involved with infections in the oral cavity: an in vitro study.
  3. Spices, oregano, dried Nutrition Facts & Calories.
  4. The antibacterial activity of oregano essential oil (Origanum heracleoticum L.) against clinical strains of Escherichia coli and Pseudomonas aeruginosa.
  5. Antibacterial activity of oregano (Origanum vulgare Linn.) against gram positive bacteria.
  6. Antimicrobial activity of essential oils of cultivated oregano (Origanum vulgare), sage (Salvia officinalis), and thyme (Thymus vulgaris) against clinical isolates of Escherichia coli, Klebsiella oxytoca, and Klebsiella pneumoniae.
  7. Combination of Carvacrol and Thymol: Antimicrobial Activity Against Staphylococcus aureus and Antioxidant Activity.
  8. Antiviral efficacy and mechanisms of action of oregano essential oil and its primary component carvacrol against murine norovirus.
  9. Inhibition of herpes simplex virus type 1 by thymol-related monoterpenoids.
  10. Anti-inflammatory and anti-ulcer activities of carvacrol, a monoterpene present in the essential oil of oregano.
  11. Effects of a Combination of Thyme and Oregano Essential Oils on TNBS-Induced Colitis in Mice.
  12. Origanum Majoranum Extract Modulates Gene Expression, Hepatic and Renal Changes in a Rat Model of Type 2 Diabetes.
  13. Methanolic extract of Origanum vulgare ameliorates type 1 diabetes through antioxidant, anti-inflammatory and anti-apoptotic activity.

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