Most of the molecules of substances used for medical treatments are too large to pass through the blood-brain barrier filter. Some of the smaller molecules get through, but not the whole suite as intended.
It seems that in order to cross the blood-brain barrier, only molecules less than 800-1000 atomic mass units (AMU) in molecular weight can get through. Lipid solubility seems to be another factor that facilitates passing through the blood-brain barrier. Water-soluble molecules don’t usually penetrate into brain tissue, even when very small. The molecules of essential oils are all not only small but lipid-soluble as well.
In fact, when it comes to essential oils, small molecules ( less than 500 AMU) are what they are made of. That is why they are aromatic. The only way for something to be aromatic is for the molecules to be so small that they readily leap into the air so they can enter our noses and be detected as odour and smell.
That is why oils for cooking or massage, such as corn, peanut, sesame seed, walnut, almond, olive and other oils pressed from seeds are not aromatic. They have a smell, but you can’t smell them across the room in minutes as one can when you open a bottle of Verbena, Mint, Lavandin, Cinnamon, Basil, Tea tree, Frankincense.
Essential oils of every species cross the blood-brain barrier. This makes them uniquely able to address disease, not only from a physical level but from a more basic and fundamental level – that of the emotions which are often the root cause of physical illness.