It is found in all plants as an intermediate in the biosynthesis of lignin, one of the principal components of biomass and its residues.

Caffeic acid can be found in the bark of Eucalyptus globulus[3] the barley grain Hordeum vulgare and the herb Dipsacus asperoides.

[10] It is also quite high in the South American herb yerba mate (150 mg per 100 g based on thin-layer chromatography densitometry[11] and HPLC [12]).

[22] Caffeic acid has a variety of potential pharmacological effects in in vitro studies and in animal models, and the inhibitory effect of caffeic acid on cancer cell proliferation by an oxidative mechanism in the human HT-1080 fibrosarcoma cell line has recently been established.

Caffeic acid outperformed the other antioxidants, reducing aflatoxin production by more than 95 percent.

The studies are the first to show that oxidative stress that would otherwise trigger or enhance Aspergillus flavus aflatoxin production can be stymied by caffeic acid.

[25] Oral administration of high doses of caffeic acid in rats has caused stomach papillomas.

[25] In the same study, high doses of combined antioxidants, including caffeic acid, showed a significant decrease in growth of colon tumors in those same rats.

[27] More recent data show that bacteria in the rats' guts may alter the formation of metabolites of caffeic acid.

[30] Caffeic acid may be the active ingredient in caffenol, a do-it-yourself black-and-white photographic developer made from instant coffee.