Taking HP together with red yeast may reduce the effectiveness of the red yeast supplement. Red yeast contains monacolin K, a compound that is identical to the cholesterol-lowering drug lovastatin, a 3-hydroxymethylglutaryl-CoA (HMG-CoA) reductase inhibitor. Lovastatin and other statins (e.g., simvastatin) are metabolized by the enzyme CYP3A4, which is induced by St. John's wort,[1] leading to a decrease in the serum concentration of these drugs. As a result, it is hypothesized that combining HP with red yeast may reduce its effect.[2]
HP may also interact with 5-hydroxytryptophan (5-HTP) supplements. Both HP and 5-HTP can increase serotonin levels in the brain, and therefore taking them together may increase the risk of serotonin toxicity. This interaction has not yet been verified by in vivo or in vitro studies. However, until more research is conducted, it is generally discouraged to combine HP with 5-HTP unless under medical supervision.