The Universal Claim with the Lack of Scientific Evidence
There seems to be a lot of media making hay out of this idea that synergy between cannabinoids and terpenes observed by the entourage effect can be explained by terpene’s ability to alter the permeability of the blood brain barrier (BBB). Specifically, sources in the lay-media are pointing fingers at β-myrcene, a linear (non-cyclic) monoterpene, as the main superhero in these comics.
Its name derives from Myrcia sphaerocarpa, a Brazilian shrub used in traditional medicine for treating diabetes, diarrhea, dysentery, and hypertension.  It’s the primary monoterpene in many cannabis cultivars, with levels ranging from 0.04% to 1.9% of total content.  Its increased presence is what allegedly allows cannabinoids more effective entry into the brain and central nervous system, and explains the observations that medicinal cannabis plants or full spectrum extracts exerts a wider range of effects than isolated cannabinoid therapies.
Figure 1. Chemical structure of β-myrcene, a linear (non-cyclic) monoterpene.
While explanations like this sound neat, and provide a testable hypothesis, the fact remains that this circulating piece of info is nothing but rumor. An extensive, exhaustive search of the literature revealed no peer-reviewed studies or published work of any kind which demonstrates the capability of myrcene to alter the permeability of the BBB. Recent publication Nutraceuticals agrees with our assessment as well. 
Although there are dozens and maybe hundreds of online and even print pieces that are circulating this rumor, it remains unfounded to it’s very core. Which is a shame, because other terpenes have been shown to have this selective activity against the BBB.  Borneol, a bicyclic monoterpene with an earthy scent, was shown in the previous two years to have the exact effect which has for some reason been attributed to myrcene. [5,6] The positive attention of this very wonderful effect is warranted, but let us at least make sure it is focused in the correct direction!
This scenario once again goes to show that in the age of the internet, it is imperative to demand rigor in both journalism and science. Anyone can make scientific-sounding claims. What’s worse is that those claims can be distorted, embellished, or just false. And while the anecdotal assertions that eating ripe mango makes you higher may be harmless, and even worth trying, the take home message is this: despite many people perpetuating this idea, there is not a readily available source that bolsters the claim with the results of experimentation.
The cannabis plant is magnetic. Millions of people are drawn to its allure, with more coming every day. The word cannabis is on the tips of the masses’ tongues. Thus, people need accurate information and not perpetuated verbiage littering the internet. Investigate things yourself if you have to. But don’t just believe something just because you read it online. (And if you happen across that elusive scientific reference to myrcene and the BBB, send it our way.)
- Ulbricht, Catherine. “Focus: Diabetes”. Journal of Dietary Supplements. 2011; 8(3): 239-256 [Times cited = 1, Journal impact factor = 1.070]
- Fischedick, Justin T., et al. “Metabolic fingerprinting of Cannabis sativa, cannabinoids and terpenoids for chemotaxonomic and drug standardization purposes”. Phytochemistry. 2010; 71(17-18): 2058-73 [Times cited = 118, Journal impact factor = 2.547]
- Hartsel, Joshua A., et al. Nutraceuticals: Efficacy, Safety and Toxicity. Chapter 53 – Cannabis sativa and Hemp. Academic Press, 2016. Pages 735-754.
- Zhang, Q. et al. “Borneol, a novel agent that improves central nervous system drug delivery by enhancing blood–brain barrier permeability”. Drug Delivery. 2017; 24(1): 1037-1044 [Times cited = 20, Journal impact factor = 4.843]
- Yu, B. et al. “The mechanism of the opening of the blood–brain barrier by borneol: A pharmacodynamics and pharmacokinetics combination study”. J Ethnopharmacology. 2013; 150(3): 1096–1108 [Times cited = 67, Journal impact factor = 3.115]
- Wu, T., et a. “The Role and Mechanism of Borneol to Open the Blood-Brain Barrier”. Integr Cancer Ther. 2018; 17(3): 806–812 [Times cited = 1, Journal impact factor = 2.675]
Image Citation: Neuroscientifically Challenged