Millions may be taking potentially dangerous combinations of herbal and conventional medicines, pharmacists have warned.
People do not admit to taking herbal remedies
Researchers told the British Pharmaceutical Conference in Harrogate many people do not view herbal remedies as medicines.
Two thirds did not admit to taking herbal medicines when they collected prescriptions.
Phamacists said more awareness was needed of the risks of combining drugs and remedies.
Researchers from the Department of Pharmacy at Kings College, London, questioned 929 people visiting four pharmacies in West London in the study.
They found that, even when people were asked what medicines they were taking, 41% did not mention herbal remedies because they did not class them as medicines.
Seven per cent of those studied were taking potentially dangerous combinations of herbal remedies and prescription medicines.
The most common was taking the remedy St Johns Wort along with Selective Serotonin re-uptake Inhibitors (SSRI's).
St John's Wort is taken for mild to moderate depression and should not be taken with prescribed antidepressants because the herb can make the drug break down more quickly in the body.
If St John's Wort is taken with the contraceptive pill, it can cause nausea, vomiting, headaches and anxiety.
Other potentially dangerous combinations include taking ginkgo, which can causes haemorrhages when taken with aspirin, and combining ginseng with warfarin, which can dangerously thin the blood.
Swati Patel, the pharmacist who led the research said the study showed how important it was for patients to tell pharmacists if they were taking complementary medicines.
"The results clearly show that many patients on prescribed medicines perceive complementary and alternative medicines to be natural and safe and do not associate any adverse drug problems when both are taken together.
"Health professionals need to be more aware of complementary and alternative medicines taken by patients and be able to advise them on the possible side effects.
"It's also important for professionals to understand the health trends of their local population."
She added: "This study identified, for example, that people of Asian origin took the largest amount of complementary and alternative medicines, many of which were Ayurvedic medicines.
"A health professional based in an area with a high Asian mix should be ready to ask the appropriate questions when people come in with a prescription."