Anti-cancer properties of Nigella spp. essential oils and their major constituents, thymoquinone and beta-elemene.

Essential oils are the volatile fraction of aromatic and medicinal plants after extraction by steam or water distillation. They have been used for their pharmaceutical potential since early times, and even now are still subject to a great deal of attention, as is clear from the increasing number of publications each year on this subject. This review presents both fundamental and recent studies concerned with the role of Nigella species essential oils and their major constituents, thymoquinone and beta-elemene, as potential chemotherapeutic and chemopreventive anti-cancer agents. The mechanism of action and the factors which determine the concentrations of these major constituents in the essential oil are also reviewed.