Topically - As a general rule, if using essential oils topically, they should be diluted in a carrier oil of about 3-5 drops of essential oils per teaspoon of carrier oil (and much less if using on a baby or child). Undiluted use on the skin can cause irritation or an allergic reaction in some people. All essential oils, diluted, should be tested on an arm before using on a larger part of my body. Some essential oils are considered ok to use undiluted on the skin if an individual isn’t sensitive to them, but always check with a qualified practitioner first.
Internally - Essential oils should only be taken internally in situations where they are absolutely needed and with extreme care and under the guidance of a trained professional.
During pregnancy and lactation - Many oils are considered safe during pregnancy, especially after the first trimester (depending on the source), but check with a professional and use caution with any herbs used during pregnancy and lactation.
With Animals - Be particularly careful if you are interested in using essential oils on your animals. Cats are particularly sensitive to oils that contain polyphenolic compounds such as Tea tree because they interfere with their liver detoxification processes.