Berberine is a quaternary ammonium salt from the protoberberine group of benzylisoquinoline alkaloids, occurring naturally as a secondary metabolite in some plants including species of Berberis, from which its name is derived.

[4] Under ultraviolet light, berberine shows a strong yellow fluorescence,[5] making it useful in histology for staining heparin in mast cells.

[13] Berberine is known to inhibit the activity of CYP3A4, an important enzyme involved in drug metabolism and clearance of endogenous substances, including steroid hormones such as cortisol, progesterone and testosterone.

Several studies have demonstrated that berberine can increase the concentrations of cyclosporine in renal transplant patients and midazolam in healthy adult volunteers, confirming its inhibitory effect on CYP3A4.

[18] The alkaloid berberine has a tetracyclic skeleton derived from a benzyltetrahydroisoquinoline system with the incorporation of an extra carbon atom as a bridge.