Hypericin is a red-coloured -derivative, which, together with , is one of the principal active constituents of (). Hypericin is believed to act as an and non-specific . Hypericin may inhibit the action of the enzyme , leading to increased levels, although thus possibly decreasing and .
The crude extract of Hypericum has weak receptor affinity for and receptors. Isolated hypericin does not display this activity, but does have affinity for .
The large system in the molecule means that it can cause when ingested beyond threshold amounts. Because hypericin accumulates preferentially on tissues, it is also used as an indicator of cancerous cells. In addition, hypericin is under research as an agent in , whereby a biochemical is absorbed by an organism to be later activated with spectrum-specific light from specialized lamps or sources, for therapeutic purposes.