There are several herb drug interactions that can potentially develop with the use of this herb. St. John's Wort is a natural anti-depressant for mild to moderate depression. After years of concern, a recent study determined St. John's wort affects the metabolism or breakdown of at least half of all drugs, reducing their effectiveness. The study specifically looked at how the remedy interacts with two enzyme subsystems, CYP 3A4 and CYP 2D6.
These enzyme subsystems allow the vast majority of medications to work in the body. Thus, you could possibly experience a decrease in the effectiveness of any prescription medications you are taking by as much as 50%.
Do not take with other anti-depressants, HIV medications, oral contraceptives, some heart and/or blood thinning medications and the cancer drug Tamoxifen. The National Institutes of Health (NIH) conducted a study that seemed to indicate a significant drug interaction between St John's wort and Indinavir, a protease inhibitor used to treat HIV infection. Administration of St. John's Wort along with Indinavir substantially decreased Indinavir plasma concentrations.