Herb-Drug Interactions and St John’s Wort

Article & Photo by Ann Walker

According to media reports, interactions between herbs and drugs are common and threaten anyone taking prescription drugs. In reality, drug interactions with foods are more prevalent, having led to several deaths (in particular, cranberry juice with warfarin and grapefruit with blood pressure drugs). Whilst no herb-drug deaths are recorded, non-fatal herb-drug interactions occur at around four cases a year and mostly involve the drug warfarin taken with St John’s wort. Despite the rarity of records, interaction ‘lists’ are padded with speculative claims based on test-tube or animal studies.

Often the quantities of herbs used in these studies are far higher than used in clinical practice or in over-the-counter products. Actually, the likelihood of herb-drug interaction is low with normal use of most herbs taken at recommended doses. Unfounded comments and prejudices expressed by scientists often mislead journalists into publishing damaging misinformation on herbal medicine. In particular, surveys of the use of herbs do not equate with drug interactions, despite scientists’ conclusions often pointing that way.

In the case of St John’s wort, we do have clinical (human) evidence of interactions with prescribed drugs. The herb’s use for depression is well supported by clinical trials (well over 30 to date). So much so that the herb provides a very viable alternative to modern antidepressants in many situations and the side effects are much lower. However, a study published in the Lancet in 2000 gave the first indication of how the concurrent use of St John’s wort with certain drugs could lower the level of the drug in the bloodstream (PMID: 10683007).

It was later found that the herb increases particular detoxication pathways in the liver (where there are many such pathways) and thus increases the rate of elimination of drugs which are disposed of by these same pathways. Not all drugs are affected by any means, but there are some important ones that are: cyclosporin, digoxin, warfarin and the contraceptive pill. Despite these known interactions, St John’s wort is a success story. Its use for depression is well founded and quality extract are on sale to the general public, with Traditional Herbal Registration (look for THR on the pack). Each pack containing a comprehensive leaflet stating clearly when it should be not used. As long as you follow these guidelines, St John’s wort remains a very useful herb – not only is it antidepressant, but it is anti-viral, anti-inflammatory and can (alongside other herbs) help with back pain, shingles, anxiety, low vitality and insomnia. I use it a lot in my practice.

PMID = PubMed identifier

Ann Walker PhD, FCPP, MNIMH, RNutr
Course Director DHM
Herbal Practitioner