Diterpenoids are 20 carbon terpenoids composed of 4 isoprene units. Although they are widely distributed and clearly biologically active, much less is known about diterpenoids as a class. The discovery of paclitaxel, a taxane diterpenoid from Taxus brevifolia (Pacific yew), has sparked renewed interest in this family of compounds.

Diterpenoids are of sufficiently high molecular weight compared with low-molecular-weight terpenoids that they are not volatile; thus, they are odorless. They are very lipophilic and tend to have strong flavors. They are typically found in resins in plants, as is discussed later. They do not tend to occur as glycosides. Little is known of their pharmacokinetics, although presumably, they are well absorbed orally, given their lipophilicity.

Select Diterpenoid-Rich Herbs
Coffea arabica (coffee)
Montanoa tomentosa (zoapatle)
Plectranthus barbatus (forskohlii, coleus)
Taxus brevifolia (Pacific yew)
Teucrium chamaedrys (germander)*
Vitex agnus castus (chaste tree)
*Contains potentially hepatotoxic neoclerodane diterpenoids.

Diterpenoids show a wide range of activity, as do low-molecular-weight terpenoids. (Select Diterpenoid-Rich Herbs) Paclitaxel promotes tubulin assembly into stable microtubules, which tends to cause death in cancer cells. Many other diterpenoids have been shown to have antineoplastic activity, and a few isolated examples, like iridoids or sesquiterpene lactones, have bitter properties. Too little is known of the pharmacokinetics and toxicology of these compounds for general statements to be made about them.