This category is touched on only partially here because lipids are primary constituents that are most often discussed in terms of their nutritional effects. Fatty acids, contained in fixed oils, are important therapeutic elements in many plants, particularly in their seeds. Omega-3 and -6 essential fatty acids from numerous species, including flax and Oenothera biennis (evening primrose), have been repeatedly shown to be inflammation- and immune-modulating agents. The fatty acids from Ricinus communis (castor) seeds act as cathartic laxatives when taken internally but have intriguing inflammation- and immune-modulating properties and act to stimulate labor when applied topically to the skin or cervical os, respectively. Such effects generally require higher doses of the oil and thus are of little relevance in many crude plant extracts (particularly aqueous ones) that are used predominantly in modern botanical medicine.

Waxes are similar to fatty acids but have longer backbones than glycerin; in addition, the esterified hydrocarbon chains are generally much longer. These are mostly solid at room temperature, although a few, such as wax from Simmondsia chinensis (jojoba) seeds — a misnomer because it is native to the southwestern United States and northern Mexico — are liquid. These highly lipophilic compounds prevent water loss from leaf and other plant tissues. Although they generally have no known therapeutic effects, they do serve an important role in the manufacture of ointments for topical delivery of other lipophilic plant constituents, as well as for other commercial and industrial purposes (Summary of Solubility).

Summary of Solubility
Constituent ClassGeneral Solubility
GlycosidesWater (most aglycones are lipophilic)
TanninsWater (especially hot)
Low-Molecular-Weight TerpenoidsLipid**
Volatile OilsLipid
Triterpenoids and Steroidal SaponinsWater and lipid (detergent)
Alkaloid SaltsWater
Lipids and WaxesLipid

*Water-soluble constituents will generally extract into water, glycerin, and <30% ethanol.

**Lipid-soluble constituents will generally extract into nonglycerin organic solvents, lipids, and 60% to 90% ethanol.

***Extract in acidic solvent, typically with addition of a weak acid such as vinegar, converts alkaloids to their salt form or keeps them in that form.