I've been getting questions on my Facebook Group "Essential oils and internal use" about what scientific research there is on the safety and efficacy of ingestion of essential oils. As a starting place, I tried a search in PubMed, but came up with too many references to nano-encapsulation methodology.
So then I thought I would go into my EndNote library which I've been saving references into for about 20 years, and search for the word "capsule". About 50 articles turned up, so I've made a bibliographic list of these, plus a table summarising the oils or constituents used in the capsules, and the conditions the encapsulated oils were used for (see PDF at the end of the article). The list is not an exhaustive list of the research on trials using ingested encapsulated essential oils, but it gives you an idea of the trials available. In addition to my reference list, p. 50 of "Essential Oil Safety", by Tisserand & Young (2014) lists trials on: Cinnamon Bark for loss of appetite; Geranium oil for stress and hypertension; Juniperberry oil for dyspepsia; Oregano oil for intestinal parasites.
Some of the products like GeloMyrtol, Tavipec, Silexan/Lasea, Colpermin and Rowachol seem to have been allowed onto the market without very much in the way of clinical trial evidence. I imagine they are formulations firstly developed in herbal medicine and phytotherapy, which, being known to be effective, were allowed to be sold as over-the-counter preparations, much in the same way as Vicks Vaporub and TigerBalm muscle rub. In particular, I'm curious that there doesn't seem to be much work done to determine the minimum effective dose, but rather the dosages used range from 80 mg to 1200 mg of essential oil per capsule without discussion of why those quantities had been chosen. The reported benefits of the essential oil capsules appear to be moderate to good improvement of symptoms (though I wonder about publication bias, and the non-reporting of non-effective treatments). Most of the clinical trials on the list mention adverse events, if there were any, and mostly these are limited to burping, nausea and mild stomach pain.
What we need to feel confident in safe and effective internal use of essential oils is some careful research to determine the therapeutic window for each oil (and for each condition we are using the oils for). Unless we use high enough dosages, we are unlikely to see much effect (beyond perhaps a placebo effect), but if the doses are too high (or repeated too frequently or for too long), we are likely to start seeing adverse events. And for most oils, we don't know how much EO must be ingested for which effect, or at what dosage level people are likely to experience adverse events.
Herbalists, phytotherapists and aromatic medicine practitioners are more likely to be able to recommend dosages that may be effective, but even they are experimenting, as we don't have clinical studies on enough people. There is nothing stopping someone from experimenting on themselves and possibly finding much benefit, but there is not enough evidence yet for us to know what are the safe AND effective internal dosages for most of our essential oils. In my opinion, the likely risks of toxicity and ill-effects of long-term internal use are too high to warrant the home-use of essential oils in capsules, unless under medical supervision. Unless you want to offer your body to science... in which case, please document the treatment and the results and let us know!!