ABSTRACT
Terpenoids, or terpenes, comprise one of the most important groups of active compounds in plants with over 20000 known structures. The bitter iridoid monoterpenes usually occur as glycosides. Non-glycosidic iridoids include the sedative valepotriates found in valerian—Valeriana spp. Numerous sesquiterpene lactone-containing plants of the Asteraceae family are known to cause contact dermatitis in humans. In one case a patient developed acute dermatitis of the right hand with severe blistering following a single application of Arnica tincture. The term ‘bitter principle’ is used in reference to any one of a group of unrelated constituents responsible for the bitter taste characteristic of many herbs. Most are derived from terpenes, though non-terpene structures such as certain flavonoids and alkaloids are included with the bitters. The main group of interest is the carotenoids, over 600 of which are found in nature—providing red, orange and yellow pigments to fruits and vegetables.
