According to the blog of Ananda Apothecary, a cited Journal of Psychopharmacology, Oct. 11, 2010 article, sage essential oil improved cognitive function and mood.
According to the study, "oral consumption [of sage essential oil] leads to improved performance of secondary memory and attention tasks, reduced mental fatigue and increased alertness." This is done through AChE inhibition or in other words, sage essential oil helps to prevent the breakdown of the neurotransmitter acetylcholine.
According to the research cited on Ananda's blog, the ingestion of 1 drop of sage essential oil will not be harmful and in fact it states that it is well tolerated by the body.
Previous research has indicated AChE inhibition from other essential oils such as rosemary, eucalyptus and lemon. These stimulant oils have been shown to improve test scores and decrease error frequency.
Source Of Information:
Ananda Apothecary Blog post 10/20/10