There are three major ways that essential oils can enter the body: ingestion, inhalation, and absorption through the skin.

In the UK, ingestion is not widely used, although doctors and practitioners in France prefer this method since with this method every drop of essential oil reaches the body systems, as opposed to inhalation where only a tiny percentage of the oil vapors enters the body and external application where some of the oils are lost through evaporation.

Information regarding the components of an essential oil is crucial when it is taken by mouth, since there are potentially dangerous components which, although still ingestible and effective for certain conditions, ought to be carfefully monitored as to the strength of concentration, the type of diluent used, and the duration that the oil is to be taken. Most commonly, alcohol and honey water are used as the dilutants, although vegetable oils such as olive oil and hazelnut oils are preferred by some doctors and naturopaths in France who believe in prescribing essential oils by ingestion. Although each individual and each oil is unique in terms of dosage, an approximate guideline for the maximum safe dosage is three drops three times daily for three weeks at a time. Since the oil is ingested, care must be exercised as ongoing use of certain powerful oils can lead to liver toxicity. Therefore, after three weeks' use, a few days moratorium in the use of the oils is recommended so as to encourage the liver and give it time to eliminate any toxic build up. Also, ingestion requires additional caution because the oil molecules may be altered by the digestive system enzymes, acids, and metabolization.

Essential oils are often bitter-tasting and may tend to irritate the mucous lining of the mouth. Sometimes they are placed into capsule form. Essential oils may also be taken internally via suppositories and pessaries, which can be effective in healing irritable bowel syndrome, hemorrhoids, vaginal infections, and candida. Suppositories allow direct entry into the bloodstream, with the maximum dose at 6 drops. Toxic essential oils need to be avoided.