Many constituents of medicinal herbs have a low bioavailabity, especially if administered orally. They do not pass the intestinal barrier. This is particularly the case for essential oils. A study (Ryuichi Fujisaki et al., www.tm.mahidol.ac.th/seameo/2012) of the in vitro and in vivo antiplasmodial activity of 47 essential oils showed that several have strong in vitro antiplasmodial activities, with IC50 values < 1.0 µg/ml. But notably these oils showed no efficacy when administered orally.