Among all complementary approaches to healthcare, herbal medicine is probably the most popular. About one third of us try remedies such as echinacea for a cold, garlic for elevated cholesterol, valerian for disturbed sleep and St John's Wort for low mood. Even sceptics have surely enjoyed a cup of tea to revive their spirits, and strictly speaking, that too is herbal medicine. Yet most consumers have only a vague idea about what herbal medicine is. Many confuse it with homoeopathy, non-herbal food supplements or vitamins.
Herbal medicinal products are dauntingly defined by EU officials as "any medicinal product containing as active ingredients one or more herbal substance or one or more herbal preparations, or one or more such herbal substances in combination with one or more such herbal preparations". Essentially this means that herbal medicines have to be whole plant extracts rather than preparations of single chemicals contained in a plant. Aspirin, for instance, a chemical derived from willow bark, is obviously not a herbal medicine.
In many ways, a herbal extract is like a good wine, as these medicines differ according to climate, soil, time of harvest, handling of the crop, method of extraction and storage conditions. A good chateau differs from year to year; similarly herbal extracts can vary from batch to batch. Consequently, a degree of variability in quality is unavoidable.
Unlike conventional medicines, herbal remedies are not usually licensed as drugs. This lack of control means that some of the products are better than others. One brand of paracetamol is much like the next, but one brand of garlic remedy can be substantially different from another. My advice is to buy from a reputable source. New European regulation will soon provide for better consumer protection, but currently there are few guarantees.
Many British doctors insist that herbal medicine is useless. But they often lack sufficient knowledge in this area. The evidence shows that some remedies work very well and have powerful effects on our health: hawthorn improves the symptoms of heart failure; kava decreases anxiety; garlic lowers cholesterol levels; Ginkgo Biloba slows the decline of mental functions in Alzheimer's disease; and St John's Wort alleviates depression.
However, the pessimistic view of the sceptics is sharply contrasted by the frequently over-optimistic view of herbal medicine promoters who insist that their remedies are natural and hence safe. Some, perhaps even most, are reasonably safe but other herbal medicines can seriously endanger our health: Some herbs are toxic and can damage organs such as the liver or kidney. Some preparations, particularly those from Asia, are contaminated with poisonous substances such as heavy metals. Occasionally, herbal mixtures have been adulterated with drugs like corticosteroids or other powerful prescription medicines. In rare cases, one harmless herb in a mixture has been replaced by a similar-looking but poisonous plant. In all of these events, consumers can suffer serious harm.
If a herbal remedy works, shouldn't we simply isolate its pharmacologically active ingredient and produce it synthetically? Then all the uncertainty about the quality and strength of the extracts could disappear. Unfortunately, this seemingly simple strategy has been successful only in isolated cases: many years ago, chemists managed to produce digoxin (a powerful heart medication) from the foxglove, Aspirin from the willow and morphine from the poppy. More recently they identified potent anti-cancer drugs from the needles of the yew tree and from the common periwinkle. But more often than not, isolating active ingredients from herbal extracts does not work. Herbal medicines contain not one but a myriad of active compounds. Isolate one and you only have a fraction of the total effect.
This level of complexity can make research into herbal medicine difficult. Add lack of funding to the equation and you see why so many questions in herbal medicine remain unanswered. There are few big players to support expensive clinical trials. And plants cannot be patented, so the impetus for commercial investment is close to zero.
The way forward is neither over-enthusiastic promotion nor negative scepticism. What we need is well-informed realism - a depressingly rare quality in the realm of herbal medicine.
· Edzard Ernst is professor of complementary medicine at the Peninsula Medical School at the Universities of Exeter and Plymouth.