These are my thoughts about internal use:
Anything a person applies to their skin should be considered "internal" use since it potentially penetrates the skin and enters the bloodstream. Even things inhaled become "internal." If something is harmful topically or inhaled, then don't put it on the tongue or swallow.
I feel that the oils are most effective applied topically or inhaled unless they are needed in the stomach or for digestion. If you want the oils to go quickly through the liver, be metabolized and eliminated, then swallowing a capsule with a couple of drops might be the answer. When our family uses an oil "internally," we usually put a drop on the tongue and let it absorb into the mucus membranes (bloodstream). We'd do this for a cough or for the stomach bug. Otherwise, we use the oils topically or inhaled.
Kurt Schnaubelt, Ph.D.:
"The discussion about the safety and overall usefulness of ingesting essential oils has gone on for a long time; it consists mainly of a superficial back and forth of talking points provided by different parties with vested interests. Obviously those engaged in the manufacture and distribution of adulterated oils, knowing about the added natural or synthetic chemicals, will warn against ingesting essential oils. It appears, however, that ingesting a drop of such oils [authentic oils] is harmless, based on all the experience gathered in aromatherapy and from what is known about the toxicity of quite a number of essential oils." - The Healing Intelligence of Essential Oils, pages 131-132.
Schnaubelt mentions that these oils, because they are high in ketones can be toxic:
Sage officinalis, Mugwort, Thuja, Hyssop officinalis and Lavandula stoechas. Ingesting 3 drops of any of these oils constitutes an excessive dose with potential toxicity.
In the Encyclopedia of Essential Oils by Julia Lawless, the safety information for Hyssop officials is:
"Non-irritant, non-sensitizing; the oil is moderately toxic due to the pinocamphone content. It should be used only in moderation and avoided in pregnancy and by epileptics."
In the Essential Oil Desk Reference, Sage is noted for dietary use diluted 50:50, but Julia Lawless says it is an "oral toxin (due to thujone)."
As you can see, there is not a concensus of opinion even amongst noteworthy aromatherapists. The good news is that when people will use the oils as a single drop, there is not a single report of harm. Anything, when used in inappropriate amounts can be toxic.