The team analysed the results of 22 trials examining drug interactions of St John's wort. The trials (published between 1999 and April 2004) studied an average of 12 participants and most used a "before and after" design.
Three trials found no important interaction, while 17 found a decrease in the blood concentration of conventional drugs when taken alongside St John's wort.
This review shows that St John's wort has the potential to reduce the availability of many conventional drugs in the body. However, the studies are small and limited in scope and methodological quality, say the authors.
"Clinicians and patients should beware of possible reductions in bioavailability of conventional drugs when taken concomitantly with St John's wort. Higher quality research is necessary to provide reliable information to guide clinical practice," they conclude.