Long-term use of the herbal St John's wort may reduce the efficacy of at least 50 per cent of drugs taken by Americans, owing to its effect on CYP enzymes, shows a small study in today's issue of JAMA.
St John's wort, widely taken to treat depression, is known to be implicated in drug interactions. However the research shows that the extent to which it may act on the metabolism of numerous prescription medicines. The study confirms the need for health professionals to be properly educated on herbal medicines as self-medication becomes increasingly widespread among consumers.
The team from the University of South Carolina found that a 14-day course of St John's wort doubled the activity of CYP 3A4 enzyme, when tested on metabolism of alprazolam drugs.
"This suggests that long-term administration of St John's wort may result in diminished clinical effectiveness or increased dosage requirements for all CYP 3A4 substrates, which represent at least 50 per cent of all marketed medications," reported the researchers.
The researchers carried out an open-label crossover study from March 2002 to February 2003 involving 12 healthy men and women aged 22 to 38 years.
Participants were given probe drugs (30mg of dextromethorphan and 2mg of alprazolam) to establish baseline CYP 3A4 and CYP 2D6 activity. After a minimum seven-day washout period, participants began taking one 300mg tablet of St John's wort three times per day for 14 days. Subjects then took probe drugs along with 1 St John's wort tablet to establish postadministration CYP activity; the St John's wort dosing regimen was continued for 48 hours.
The researchers reported a twofold decrease in alprazolam plasma concentration and a twofold increase in alprazolam clearance after St John's wort administration.
Alprazolam elimination half-life was shortened from a mean of 12.4 hours to 6.0 hours. By speeding up metabolism of drugs, the herb prevents patients from getting the right dose of medication, causing potentially dangerous consequences on health.
This is compounded by the fact that many consumers do not consider possible interactions between herbs and drugs. In a study presented in the UK this week, researchers warned that one in 20 people could be unknowingly risking their health owing to the side effects of a combination of prescription and alternative medicines.