Pharmacokinetic and pharmacodynamic investigations are also required. Finally, the health consequences of passive vaping should be also considered.  
In conclusion, the new social phenomenon of vaping may slide from nicotine towards other psychoactive drugs (e.g., THC); it therefore deserves the urgent scientific investigation and strict risk assessments which are especially important when young people are concerned. In particular, the presence of toxic substances in the cannabis aerosols generated by e-cigs—from multiple models, brands, BHO and e-liquid manufacturers—need to be investigated.
A synthesis of the practical recipes and experiments reported on the Internet (in addition to our own experiments) shows that the simple, direct use of purified cannabis extracts in e-cigs is not easy because cannabinoids are poorly soluble in e-liquids. Indeed, used pure or in mixture with glycerol, the propylene glycol (PG, or propane-1,2-diol) that is commonly used in e-liquids has both hygroscopic and hydrophilic properties. Consequently, it is not miscible with mineral and vegetable oils and fats. In contrast, cannabinoid concentrates are very lipid soluble, but water insoluble. Furthermore, PG, glycerol, terpenoids, plant fats and waxes, cannabinoid concentrates and THC are very viscous substances. Accordingly, the manufacture of a homogeneous solution using these viscous liquids is a tedious task. Cannabinoid concentrates resemble more a thick, sticky, gummy resin than a liquid. We can infer from these facts that they do not mix readily with glycols, and even less so with glycerol. The addition of terpenoids (e.g., limonene) or lecithin has been advocated to help make cannabis oils miscible...