The answer is - maybe, sometimes, sort of, but mostly no ;)
Oregano is a culinary herb used in food. Historically the herb has been recognized as having antibacterial, antispasmodic, antiviral, calmative, carminative, diaphoretic, expectorant, stomachic, and tonic actions among many others. It has a long and safe history as a food spice/flavouring agent, tea, and whole herb infused oil. In these forms there are no established contraindications, adverse affects, or limitations on use during pregnancy or breastfeeding and it is generally recognized as safe for internal consumption when taken only in food quantities.
Most of the time however when people ask me about using oregano they’re not referring to making a whole herb tea using fresh or dried leaves, or adding some extra flavour to their food. They are instead referring to oregano essential oil, or to oil of oregano. It is important to note that oregano oil, oregano essential oil, and oil of oregano are not necessarily the same. The main problem with most articles online about “oregano oil”, is that the authors either don’t understand or fail to make clear, the difference between the pure essential oil, the diluted essential oil, and the herbal oil. Pure undiluted oregano essential oil should not be used topically or internally by anyone due to heightened risks of sensitization and adverse reactions. Proper dilution (which we will come to below) helps reduce some but not all of those risks. The second problem with many online articles is that they have a tendency to take a little information and use it out of full context which leads to either understating or overstating the risks and benefits depending on the perspective of the author.
Traditionally “oil of oregano” referred to the whole herb infused into vegetable oil. There are still some companies who make it this way but they are few and far between as best I can tell. Most companies now sell oregano essential oil (sometimes diluted in an edible plant oil like olive oil) as “oil of oregano” which doesn’t help the confusion. Popular companies such as Genestra, Joy of the Mountains, Organika, St.Francis Herb Farm, and Natural Factors all sell an “oil of oregano” product which is the essential oil prediluted in olive oil or the pure essential oil,
While oregano essential oil has a strong history and some science behind its use as a potent antibacterial and antiviral agent, potent things also tend to carry risks. Carvacrol (the primary constituent in oregano essential oil) has blood thinning properties and blood sugar lowering properties and a 2007 study showed that the primary constituent in oregano essential oil (carvacrol) caused embryonic cell death. A separate study from 2003 however showed that carvacrol in pregnancy increased implantation and live birth rates. Tisserand, who literally wrote the book on essential oil safety, states that there are no indications of reproductive toxicity from carvacrol but that it could possibly be genotoxic depending most likely on dosage. It is however a known irritant to mucous membranes and skin, and Tisserand recommends that carvacrol not be used at concentrations of greater than 1%. Most popular oregano essential oil products such as those noted above boast levels of carvacrol that are 75% or higher. Because they are sold as dietary supplements very little is required in terms of providing evidence of safety for the consumer and so the consumer must do their own research to ensure that what they are taking is safe and appropriate for them. What does all this mean in terms of safety during pregnancy and while breastfeeding specifically?
It means that there is no reason to believe that consuming the whole herb oregano in food quantities is problematic during pregnancy or while breastfeeding, BUT when you use oregano essential oil, even when prediluted and sold as an "oil of oregano" product, you increase your risk of mucous membrane and skin irritation, but more importantly your risk of bleeding and that is a risk most people don’t want to take at any stage in pregnancy as it threatens the wellbeing of both the pregnant/birthing person and the fetus. It is because of the increased risk of bleeding that I do not recommend the use of oregano essential oil products in pregnancy.
In terms of breastfeeding the risks are less clear. We do know that what we eat passes through into our breastmilk but there is little to no evidence of risks or benefits of (prediluted) oregano essential oil use while breastfeeding. It is a very powerful and potentially caustic essential oil which is known to decrease milk supply in susceptible persons. Because of its effect on clotting, and because newborns are born with less ability for their blood to clot than adults as well as immature livers, it is my belief that additional caution may be warranted when considering the use of oregano essential oil products while breastfeeding a young infant.
Sometimes when one is faced with the choice of taking a known medication with established risks or a strong herb with less established risks, individuals will choose the herbal option. You may be more comfortable with your mouth and throat stinging than with the side effect of a proposed pharmaceutical. Or you may decide that you're comfortable with an increased risk of bleeding. At the end of the day we each have to weigh the risks and benefits in light of our own circumstances. Thankfully there are many other safe options to help support the immune system during pregnancy if you decide that oregano oil isn’t for you. Echinacea, elderberries, rosehips, ginger and others can all be used to help your body get back to health during pregnancy and breastfeeding. You can also explore alternatives such as reflexology, hot foot soaks, warming socks and more here.
Recommended Resources & Further Reading
The Essential Oil Safety Guide 2nd ed by Tisserand & Young
A-Z Guide to Drug-Herb-Vitamin Interactions by Dr. Alan Gaby
Aromatherapy & Herbal Remedies for Pregnancy, Birth & Breastfeeding by Demetria Clark