Britons are spending £126m a year on herbal medicines but many are endangering their health by taking inappropriate remedies, herbalists say today.

The National Institute of Medical Herbalists, which is the world's oldest body of practising herbalists, says people are buying over-the-counter products that could end up doing them more harm than good.

Trudy Norris, the institute's president, said some people took no advice on which remedies were right for them and others risked serious side- effects by taking a cocktail of herbs with conventional drugs.

Some consumers bought poor-quality products, with the wrong balance of ingredients or containing chemical extracts in higher concentrations than in natural plants. Other remedies, particularly Chinese ones, were contaminated with steroids and heavy metals.

The warning from the institute, which was founded in 1864 and which represents 550 qualified herbalists, follows a number of alerts about the safety of herbal medicines.

Ms Norris said: "There are some obvious limitations to buying over-the-counter remedies, since herbs can sometimes cause more harm than good if used inappropriately, just like other kinds of medicine.

"While we are not against commercial herbal remedies, bought for self-medication, we do urge people ... to find out as much as possible before self-prescribing. This is particularly important if you are pregnant, taking any other form of medication, or taking over-the-counter remedies for anything other than minor ailments. It's preferable to see a qualified medical herbalist."

More than 80 per cent of all herbal medicines sold in Britain are unlicensed because the booming industry is unregulated. Last year, the Medicines Control Agency said it could give no assurances about the safety of many Chinese remedies after a number of products were found to contain "dangerous and illegal" ingredients.

Mercury and arsenic had been found in some remedies while eczema creams contained prescription-only steroids. The MCA also referred to an incident in Belgium in which scores of women had kidney failure after taking a slimming product containing Aristolochia, a herb banned in Britain but found in dozens of remedies.

Kava Kava, a remedy for anxiety, was withdrawn last December after German reports of liver damage, including one death. Scientists say kava, St John's wort, dong quai and ginkgo could interfere with drugs for heart conditions, asthma, depression and HIV. But a School of Pharmacy study showed that 60 per cent of people buying remedies had taken them with conventional drugs. Ms Norris said that a woman prescribed the anti-coagulent warfarin had also taken five herbs, including kava kava and ginkgo. Neither should be taken with warfarin.

Tomorrow the institute is launching a herbal medicine awareness week to encourage consumers to seek more advice and for manufacturers and retailers to be more responsible.

The pros and cons of natural treatments

Echinacea: An all-round immune-system tonic taken to stave off and treat infections. With habitual use it may suppress the immune system. Not recommended for patients with autoimmune diseases.

Dong Quai: This Chinese herb is useful for helping to balance hormone levels in women. However, it contains substances that may interfere with blood clotting and increase sensitivity to light.

Ginkgo: Good for stimulation of circulation, particularly for the brain. No serious or common side-effects. Occasionally results in stomach pain or headache, mild skin reactions and dizziness.

Ginseng: Helps maintain nervous system and useful for times of stress. Potential side-effects include nervousness, mild insomnia and diarrhoea. Menstrual abnormalities may occur in long term.

St John's Wort: A remedy for moderate depression. Not to be taken with other anti-depressants. In large doses, it has been known to increase tendency to sunburn, dizziness, dry mouth and constipation.

Valerian: Used to help people regain a healthy sleep pattern and calm people with high stress levels. Paradoxically, it has been linked to insomnia, mild headaches, muscle twitches and palpitations.