Essential Oils are available in both blends and singles.

Although I use both blends and singles I find myself using more blends.  I am able to target the issue and use the best oils for that problem.

Synergy is the result of combining two or more oils in such a manner that the result is more effective than the those oils used separately. When oils are mixed together in the correct order and in the correct proportions, a chemical compound is created that is very particular and very powerful; an increased potency can been achieved without the need to increase the dosage.  Blended oils also have the added advantage of being less likely to cause a reaction or any kind of problem.  The use of good blends raises the effectiveness of essential oils to entirely new levels.

Blends

I do use some single oils,  eucalyptus and peppermint smell wonderful diffused into the air.  I also almost always go to sleep with lavendar in the air.  There are many wonderful single oils that promote healing. Many of them, however, have lists of cautions – particularly when being used on pregnant women, children, the elderly, or people with seizure disorders.  To be fair, the majority of problems with essential oils are the result of a personal allergy or have occurred when the oil was taken internally.  Single essential oils have been used safely for centuries by reasonably cautious people who have tried to educate themselves.

Singles