Jane is 26 and has just finished her NQT year. Her work teaching biology in an inner London comprehensive is tough and demanding, but just as the job was beginning to fall into place, her long-standing relationship with Ceri came to an end. She lost confidence - both socially and in the classroom - and, feeling low, retreated from interacting with other people.

A colleague suggested she ask her doctor for Prozac, but her sister gave her a bottle of St John’s Wort tablets. The label on the bottle read:

“333mg standardised herb extract... equivalent to 1mg per tablet. The Hypericum perforatum plant... has been used in Europe for hundreds of years.” Jane threw the Prozac suggestion out the window and the St John’s Wort bottle, still sealed, went into the bathroom cupboard.

Three months later, she was still feeling low and couldn’t seem to nudge herself out of her inertia. Going to work was a huge physical and mental effort. Still sceptical, she started on the St John’s Wort. After two weeks of daily tablets, she was beginning to feel considerably better and more positive. Six months on and she was nearly back on track. After a summer holiday in Crete, some excellent GCSE results and warm congratulations from her colleagues, she started a new school year without the pills.

Jane’s experience might seem too good to be true, but her experiment with herbal remedies is part of a growing national trend. According to researchers from the Research Council for Complementary Medicine in London writing in the British Medical Journal at the end of last year, we now spend more than pound;40 million on herbal products in the UK. These products were once confined to health food shops and sold as food supplements, or were available from a herbalist (in the UK, Chinese herbalism is the most prevalent).

Now they are sold in most pharmacies, even though the vast majority of products remain unlicensed. But professional herbalists have started to voice their concern about the lack of advice available to consumers. Herbal remedies can contain potent substances. After all, until a century ago, most drugs were plant-based, and many of today’s conventional drugs were developed from plant extracts. Aspirin comes from willow bark, cocaine from the coca plant, morphine from the opium poppy and digoxin, used for treating cardiac failure, from the foxglove. Now more that 80 per cent of people use botanical preparations as medicines worldwide, and in most industrialised countries herbalism is experiencing an unprecedented rennaissance.

But are herbs as harmless as many people believe? As with conventional over-the-counter (OTC) drugs, it is not so much their use which can cause problems, but their abuse. Because they are thought of as natural, there is a mistaken belief that if a little is good, then a lot is even better. However, prolonged and excessive use of herbal supplements can provoke the same toxic and allergic reactions that conventional medication can. Herbs also interact with other drugs, and the combinations can cause nasty side-effects.

Herbal medicines may also be contaminated or adulterated with potentially toxic substances as regulation is patchy - particularly outside Europe and North America.

Thousands of herbal remedies are now on the market: 234 plants with alleged medicinal properties are listed in just one health guide available on the Internet. Ally Broughton, head of research at the National Institute of Medical Herbalists in Exeter, says that St John’s Wort, garlic, gingko, echinacea, feverfew and milk thistle extracts are among the most popular OTC herbal remedies in Europe.

St John’s Wort and the prescription-only drug Prozac are believed to help depression by increasing the availability of brain chemicals that help to increase feelings of well-being. Quite how it works is now of increasing interest because it has been so effective, but previous clinical trials have shown that it can alleviate mild to moderate symptoms of depression without the side-effects of prescription-only drugs. In Germany, where there is the greatest use and acceptance of herbal remedies, St John’s Wort has now been formally approved for clinical use. According to some sources, prescriptions in Germany for the herbal remedy outnumber prescriptions for Prozac by 25 to 1, and make up at least 25 per cent of GP prescriptions for depressive illness.

Yet it is worth bearing in mind that vets classify Hypericum perforatum as toxic because of reactions in some livestock when the plant is a major source of food. While rare, there have been incidences of rashes, unexpected sunburn or stinging pain in the exposed skin of men and women taking hypericum.

Studies are now looking to see if ginkgo, and extract from the Ginkgo biloba tree, can help alleviate the symptoms of Alzheimer’s disease - it has recently been approved in Germany for treating dementia. Originally called the “memory tree”, it is traditionally thought to help memory and cognitive function. Feverfew also acts on the brain, but its main therapeutic claim is to reduce the frequency and severity of migraines. Echinacea is one of the best selling remedies in the current herbal renaissance. Extracts are said to boost the immune system; in Europe it is widely used to protect against and to treat winter colds and flu. But it presents a real danger to those with immunity related diseases.

Claims are made for most, if not all, of these herbal remedies, that they help a wide range of conditions ranging from hepatitis to pre-menstrual syndrome to appetite loss. But by law they are still not allowed to make any specific health claims - apart from general maintenance of good health. But because they are marketed as supplements, they do not have to undergo the strict scrutiny required for conventional OTC drugs. Published side-effects can range from liver and kidney damage to irregular heart beats, although adverse reactions are generally low - “certainly much lower than conventional drugs”, according to Ally Broughton.

The problem is not just with herbal remedies. The vitamins and mineral supplements business is booming too: sales from chemists have increased by 200 per cent over the past 10 years. And as with herbal remedies, many people mistakenly believe that they are harmless because they are natural components of food.

There are 13 known vitamins. Four - A, D, E and K - are absorbed with dietary fat. The other nine are water soluble and include the B group (1, 2, 3, 6 and 12), folic acid, biotin, pantothenic acid and vitamin C. Vitamin B6 has been advocated as a treatment for pre-menstrual syndrome and the nausea and vomiting of pregnancy; vitamin E to protect against arteriosclerosis, cancer, ageing and skin diseases; the list is endless. Currently the most important benefit claimed for vitamins A, C, and E is that they are anti-oxidants and can eliminate free radicals (unstable particles which are by-products of many of the body’s chemical process and are thought to contribute to a number of diseases including cancer and heart disease, as well as the ageing process).

While we know what vitamins do and that a dietary deficiency can have some quite serious effects, it is hard to substantiate (scientifically and clinically) the claims made for the benefits of vitamin supplements. Taken in excess, some minerals and the fat soluble vitamins, which can accumulate in the body, may have toxic effects. This is rare, provided the vitamins are not taken in large doses, but there are many examples of adverse effects. Overdosing on vitamin D or calcium supplements can result in high concentrations of calcium in the blood. This is then deposited in soft tissues and can cause problems, such as kidney stones; an excess of B6 is reported to damage peripheral nerves.

Whether or not extra vitamin and mineral supplements provide any additional benefits on top of a diet that provides adequate amounts of all these micro-nutrients is open to debate. The same is true of how much of the supplements we can actually absorb and use.

And packages and bottles of vitamins will soon have to carry a health warning that too many may seriously damage your health. Such rules have yet to be approved by the European parliament and EU health ministers.

Saffron Davies is a senior lecturer at St George’s Hospital Medical School, south London