• Several suspected interactions between the commonly-used anti-depressant St. John’s Wort (Hypericum perforatum L.) and other drugs have been identified
• A number of women taking oral contraceptives at the same time as products containing St. John’s Wort reported pregnancies.
• Since St John’s Wort inhibits the synaptic reuptake of monomines, (e.g. serotonin), it may contribute to serotonin syndrome if combined with drugs that increase serotonergic activity (SSRIs)
• St. John’s Wort was shown to lower the serum concentration of the anti-HIV drug Inidavir (Crixivan), which may reduce the effect of the drug and even increase the risk of the HIV virus developing resistance
• The herb was also shown to reduce the serum concentrations of warfarin and some statins, including simvastatin and atorvastatin.
• A few cases indicated that St. John’s Wort reduced the effect of cyclosporin and caused rejection of transplanted hearts

Follow-up studies and adverse event reports revealed that many of the interactions of St. John’s Wort were serious or potentially serious. Furthermore, if these interactions had been found at that time, the conclusions drawn about the use of the respective drug might have been different.

These cases illustrate the importance of knowing as much as possible about the concomitant herbal medications taken when adverse events are detected. Today, most drugs that risk interactions have precautions about the concomitant use of St. John’s Wort added to their labeling and SPCs.