But given the increasing use of herbals and self-treatment, higher quality research is necessary to provide reliable information for doctors, they claim.
In a Mintel poll of some 25,000 consumers in the UK, over 40 per cent said they took dietary supplements, compared with around a quarter in Germany, France, Italy and just one in 10 in Spain.
This has prompted concern from the medical community as patients tend not to tell doctors about their supplement use. Researchers at the department of Pharmacy at Kings College, London, said last year that one in 20 people are taking potentially dangerous combinations of herbal and prescription medicines. The most common herb drug interaction in the study was St John's Wort with selective serotonin re-uptake inhibitors, taken by 30 per cent of those surveyed.
Other potentially dangerous interactions include taking gingko with aspirin , which can lead to haemorrhage and taking St John's Wort with the contraceptive pill, which can cause nausea, vomiting, headaches and anxiety. Ginseng can also be dangerous when taken with the blood thinning drug Warfarin.
But when an international team from the US, UK and Canada analysed the results of 22 trials examining drug interactions of St John's wort, they found that most were small and limited in scope and methodological quality.
The trials studied an average of 12 participants and most used a 'before and after' design that lacked contemporary controls or randomisation to control for carryover effects and time dependent metabolic variability, for example.
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.
"Our review uncovers several threats to the validity of pharmacokinetic studies that examine herb-drug interactions," write the researchers (BMJ, vol 329, pp 27-30). "Higher quality research is necessary to provide reliable information to guide clinical practice."
However they added that trials that include adjustments in the dose of the conventional drug, to achieve optimal area under the curve in the presence of herbs that reduce bioavailability of that drug, could offer a solution for patients wishing to use both.