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Side Effects of the Oregano Leaf

author image Tess Messer
Tess Messer is a physician assistant and freelance writer with a master's degree in public health. She has been writing since 1982 and has published clinical articles in The Journal of the American Academy of Physician Assistants and online. She lives with her spouse and two boys in Ashville, N.C.
Side Effects of the Oregano Leaf
Oregano use may cause side effects. Photo Credit: compuinfoto/iStock/Getty Images

The oregano leaf has been used for thousands of years for culinary and medicinal purposes. Turkish, Spanish, Mexican and Greek oregano contain different amounts of carvacrol, thymol and other active ingredients. Origanum Vulgare from Greece is thought to have the greatest medicinal value. According to, oregano is safe when used as a food but should be used with caution if you are pregnant, diabetic, have sensitive skin or if you have plant allergies.

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Skin Sensitivities

The two main active ingredients in Oregano are the phenols carvacrol and thymol. Dr. A. Anderson in the 2006 supplement of the “International Journal of Toxicology” reports that animals exposed to the oregano leaf rarely developed skin irritations. However, the Environmental Working Group reports that these substances are easily absorbed and can cause skin irritation, especially in infants, children and with exposure to mucous membranes such as the eyes or nostrils.

Thymol Toxicity

Thymol is one of the active ingredients in the oregano leaf. Some species of Mexican oregano, or Lippea Graveolens, have considerably more thymol than Greek oregano, or oregano vulgare, whose primary component is carvacrol. Thymol has been used as a pesticide and the Material Safety Data Sheet reports that chronic exposure to thymol can cause damage to the Kidneys, liver, central nervous system and mucous membranes.

Allergic Reactions

People who have allergies to other herbs should use caution before consuming oregano. The oregano leaf is in the labiatae family and can cause severe allergic reactions in individuals who are allergic to sage, mint, basil, lavender and marjoram, which are other herbs in the labiatae family, according to Dr. M. Benito in the “Annals of Allergy, Asthma and Immunology."

Pregnancy Side Effects

Pregnant women should use oregano with caution.
Pregnant women should use oregano with caution.

Inadequate levels of iron during pregnancy are known to cause problems with the developing fetus. Pregnant women should limit oregano consumption because the phenols in oregano may reduce iron absorption. Another reason to avoid oregano during pregnancy is because it is a phyto-estrogen that has been used in traditional medicine to terminate pregnancies and to bring on menstruation. Oregano is safe when used as a food additive but that large dosage of oregano should be avoided during pregnancy.


The Glucosides found in the oregano leaf can lower blood sugar, reports the January 2008 “Journal of Natural Medicine.” Diabetics who are on medication to lower their blood sugar should use caution when using oregano because of the possibility of hypoglycemia.

Gastrointestinal Upset

According to CAMline Reviews, oregano can occasionally cause gastrointestinal upset if taken in high doses. No reported complaints of stomach upset have occurred when using oregano to spice foods.

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