As an herbalist trained in the use of botanical medicine, I am often floored and upset by the ways people tell me that they have used essential oils. And I am infuriated when they tell me that what they learned is all over the internet or that an MLM sales rep (not trained in biochemistry) tells them to use them in an unsafe way.
Frankly, there are many memes like this one from Young Living, DoTerra and other unscrupulous brands that are unsafe and wrong:
How many ways is this meme suggesting things that are unsafe? Let's see.. wrong population and oil choices many singles and blends listed here are unsafe for children under the age of 10), no dilution, improper dilution into liquid that do not dilute Eos (vinegar and water) and improper application (to the bottoms of feet).
These memes are all over the internet, leading people to believe that they are safe because they have seen them over and over. Bottom line: Do not take medical advice from a sales rep.
There are two safe ways to use essential oils for laypeople:
1. inhalation (smelling them) and
2. topical application (always properly diluted in a carrier of either oil or alcohol). 
Any other route you have heard of is unsafe. Here are some I have encountered that are just plain wrong:
Ingestion: Do not put essential oils in beverages, recipes, or directly in the mouth. Corrosive substances like EOs should not come in contact with mucus membranes, ever. And your entire digestive tract is mucus membrane.
Tampons, soaked in EO to get rid of yeast infections or shrink hemorrhoids: Again, no contact with mucus membranes. This is not only dangerous and painful, but could result in permanent damage and scarring. Please do not do this and if you are an MLM rep advising this, CEASE AND DESIST. You are hurting people.
Undiluted topical application: Not only are you risking sensitization (overexposure from too much or too often), you are possibly harming yourself and wasting money. All EOs should be diluted, quite a lot. It should also not be on large surface areas of the skin. If you are making a massage oil (a large surface area and exposure), your total EOs in your oil should be no more than 1%.
Diffusing for hours on end: You should really only diffuse EOs for a few hours at a time. Too much exposure risks sensitization. And never diffuse in a home with companion animals other than dogs. It is not safe for cats or birds and other exotics.
Using them for "wellness" instead of as a remedy: These are very effective when used for specific applications- hornet stings, burns, insomnia. But they are not tonics, meant to strengthen systems and make you well. Nope, they just are not. They are too concentrated for that. They are to be used sparingly, when the situation arises.
In the bath, on a bath product without being diluted in alcohol or oil first, or in a foot soak pedicure basin: EOs are hydrophobic, meaning they bead up and do not dilute in water and similar liquids. When making bath salts, you cannot simply drop in EO on the crystals and expect that they are properly diluted. Putting drops of EO in a bath will just result in concentrated exposure as it comes into contact with skin undiluted.
Using them on the wrong people: Vulnerable populations such as children, the elderly, pregnant women, and the immuno-compromised is not advisable. You should avoid using Eos with these folks as much as possible. Herbs are much safer. (And if I hear that story about Frankincense curing cancer one more time, I am gonna pitch a fit. Frankincense protects cell walls- ALL cells, including cancer cells. As someone who volunteers at Cancer Support Community every month and knows people suffering with cancer, it infuriates me how MLM companies have spread this misinformation to profit from sick people. (AND, the lab studies they quote were not on Frankincense EO, it was on Frankincense tincture, a very different product, which has a broader base of constituents than an EO. So, they just spread deliberate lies.)
When you use essential oils impro
perly, you can harm yourself and others. You risk sensitization, which is a sort of environmental illness that is created from overexposure to intense chemicals. You will start to have skin, respiratory, or neurological reactions at each exposure once you reach that point- asthma attacks, seizures, and chemical burns are not worth it.
To the left, you see a skin reaction to undiluted Rosemary EO.
Bottom line: consult a certified aromatherapist or someone like me, tried in botanical medicine for health and wellness advice. Not a sals rep trying to sell you something (who gets their info from a company deliberately misleading them).
Part One of this blog series talked about EO safety myths and who perpetuates them.
Part Two of this blog series talked about the safe way to use EOs.
Part Three talks about how to select a good brand of EO, just by looking at packaging, websites, and marketing.