Hypericin

Hypericin is a naturally occurring anthraquinone-derived pigment, structurally a phenanthroperylene quinone (a naphthodianthrone), responsible for the deep red color of St. John's Wort extracts and the reddish stain the plant leaves when crushed. It is found in Hypericum perforatum and related species, formed biosynthetically through the dimerization of emodin. Along with hyperforin, it is one of the marker compounds used to standardize herbal preparations of the plant ^[1].

Historically hypericin was described as an inhibitor of monoamine oxidase and credited with the antidepressant action of St. John's Wort, but later studies could not confirm meaningful monoamine oxidase inhibition at realistic concentrations, and the antidepressant effect is now attributed largely to hyperforin, with hypericin regarded as a lesser contributor ^[1]. Modern interest in hypericin centers on its light-activated properties. It is one of the most powerful natural photosensitizers, and hypericin-mediated photodynamic therapy has been studied extensively against many tumor types, from bladder and skin cancers to thyroid and blood cancers, where light exposure triggers it to kill cancer cells ^[2][3]. Its natural fluorescence has also been exploited to help detect bladder cancer during endoscopy, and it has been investigated for antiviral activity.

Hypericin reaches the body chiefly through St. John's Wort supplements, which are sold over the counter as herbal products in many countries; purified hypericin for photodynamic and diagnostic use remains largely investigational. The compound's light reactivity is also its main safety liability: high intake can cause photosensitization, an exaggerated sunburn-like reaction of the skin and eyes on exposure to sunlight, an effect long recognized in grazing livestock that eat Hypericum. Standardized extracts are commonly labeled by their hypericin content, and users are generally advised to limit strong sun exposure.

The main practical concern is photosensitivity, so users should limit strong sun exposure, especially at higher doses. As part of St. John's Wort it shares the herb's serious drug interaction profile driven largely by hyperforin. Isolated hypericin's long-term safety in humans is not well established.