First of all, essential oils seem to go hand-in-hand with herbs, massage, and healthy lifestyles. They're from plants, so the idea is that they're natural and safe. In many places I see people advising the use of essential oils without the warnings and distinctions that are very important, in the same way they might recommend spinach or apples. Not everything natural is safe. After all, cyanide and nightshade are natural!
Essential Oils are Drugs
The volatile oils of plants are isolated, distilled, and processed to give what we call an essential oil. The important thing to remember is that they are very concentrated. This isolating a single component from a plant is the same way aspirin, cocaine, and codeine are made, yet no one would argue that they are not drugs. Just because they are found in tiny amounts in plants obviously does not mean they are safe. My rule on using drugs is that they need to be proven to be safe and effective for the problem they're used for. While essential oils may or may not be safe in small amounts, we know they are definitely not safe in larger amounts. People have died from ingesting as little as 1 teaspoon of these powerful drugs.
Essential Oils are Not Regulated
The FDA does not approve or regulate essential oils in products like cosmetics. Manufactures can make any claim they like about them and often do. Words like, "therapeutic grade", "100% pure", and "natural" have nothing to back them up at all. This is why I think you find essential oils in many products these days. They smell good, so they are often used in shampoos, lotions, and creams without any safety guarantee. Because of this, many people may be using essential oils at levels they're not aware of.
Essential Oils are Antibacterial
One oft-repeated benefit of essential oils are that some are antibacterial, including some of the more popular. That's great, right? No one wants yucky old bacteria hanging around. Wrong! Please read my article on beneficial bacteria. Many health problems are created or worsened by the killing off of our beneficial bacteria. This is why antibiotics should be used judiciously, yet the same people wary of antibiotics sometimes take essential oils internally. This is quite alarming to me, because killing bacteria in your gut can leave space for harmful bacteria to take over.
Essential Oil Experiment
This is not a scientific study, but is a good visual to show how potent -and potentially harmful- essential oils can be. I often make homemade yogurt from my goats' milk, and it has never failed yet. Warm milk, add yogurt, then let it sit for about 12 hours while the bacteria grows and turns the milk into thickened yogurt. With this batch I took a scoop of the fresh yogurt into two bowls. I added one drop of lavender oil to one bowl, and one drop of tea tree oil to the other. These are two of the "safest" most common oils. After 12 hours it was easy to see that these oils are strongly antibacterial to the good bacteria in our bodies. The yogurt I didn't add any oils to turned to yogurt just fine. Could your essential oils be negating the effects of your probiotic food?
I've found many claims on the internet that essential oils are safe, yet most of the links I follow lead to companies that sell essential oils. Of course they downplay the well-established side effects some can have. They are meant to be used a DROP at a time. Remember, that drop represents many plants. Even if you choose to use essential oils, they are as individual as the plants they come from. Cucumbers can go on your eyes, but you wouldn't put jalapeños on them! Make sure you know which essential oil you're using and what side effects they might have. Some cause allergic reactions, some photosensitivity, and some gastrointestinal upset, to name a few. Since this website is directed at pregnant women, please be aware that some essential oils may cause premature contractions, and effects of oils on your baby is unknown. Also, please make sure your essential oils are kept away from children, since they are more sensitive than adults. Many of those documented to have been harmed by essential oils were children.
You may also want to look at possible esterogenic effects of lavender EO and some other considerations.