Hypericin is a red-coloured anthraquinone-derivative, which is one of the principal active constituents of Hypericum (Saint John's wort). Hypericin is believed to act as an antibiotic and non-specific kinase inhibitor. Hypericin may inhibit reuptake of dopamine β-hydroxylase, leading to increased dopamine levels, and is commonly used for treatment of mild depression.

The large chromophore system in the molecule means that it can cause photosensitivity when ingested beyond threshold amounts. Because hypericin accumulates preferentially on cancerous tissues, it is also used as an indicator of cancerous cells. In addition, hypericin is under research as an agent in photodynamic therapy, whereby a biochemical is absorbed by an organism to be later activated with spectrum-specific light from specialized lamps or laser sources, for therapeutic purposes.