Eucalyptol, a natural epoxide monoterpene, was found in rat brain tissue after oral administration in our previous study, suggesting that the compound may possess the ability to pass the blood–brain barrier (BBB). However, a few studies have demonstrated that eucalyptol does penetrate the BBB. The aims of this study were to determine the opening effect of eucalyptol on the BBB in rats, to establish and validate a method for the determination of eucalyptol in brain tissue, and to reveal its brain pharmacokinetic characteristics. The opening effect of BBB was assessed by dye extravasation and ultrastructural alterations, and the quantitative method of eucalyptol in rat brain tissue was established and confirmed. For pharmacokinetic research, rat brain samples were taken at 0.05, 0.167, 0.5, 1, 2.5, 5, 10, and 15 h after administration. There was a significantly higher extravasation of Evans blue from the brain parenchyma of rats in the medium‐dose eucalyptol group (P < 0.01), which was associated with the BBB's altered ultrastructure. It is suggested that eucalyptol increased the permeability of the BBB. After oral administration, eucalyptol was quickly absorbed by the brain. This study provides valuable information on eucalyptol use to treat illnesses of the central nervous system.

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The stratum corneum continues to pose the biggest obstacle to transdermal drug delivery. Chemical penetrant, the first generation of transdermal drug delivery system, offers a lot of potential. In order to fully examine the permeation mechanism of 1,8-cineole, a natural monoterpene, this review summarizes the effects of permeation-enhancing medications on drugs that are lipophilic and hydrophilic as well as the toxicity of this substance on the skin and other tissues. For lower lipophilic drugs, 1,8-cineole appears to have a stronger osmotic-enhancing impact. An efficient and secure tactic would be to combine enhancers and dose forms. 1,8-cineole is anticipated to be further developed in the transdermal drug delivery system and even become a candidate drug for brain transport due to its permeability and low toxicity.