Mesembrine is an alkaloid present in Sceletium tortuosum (kanna). It has been shown to act as a serotonin reuptake inhibitor (Ki = 1.4 nM), and more recently, has also been found to behave as a weak inhibitor of the enzyme phosphodiesterase 4 (PDE4) (Ki = 7,800 nM). As such, mesembrine may contribute to the antidepressant effects of kanna. The levorotary isomer, (−)-mesembrine, is the natural form. Rat studies have evaluated effects of kanna extract, finding analgesic and antidepressant potential. No adverse results were noted for a commercial extract up to 5000 mg/kg daily.