Bitter sweet: The special London garden dedicated to herbs

Some can cure cancer, others lift depression, but as Jeremy Laurance, herbs can also be deadly

Our generation swallows more medical pills than any in history. At the latest count, 16 prescriptions are written each year for every man, woman and child, more than a 60 per cent increase in a decade. And that doesn't include the pills, potions and treatments we buy over the counter. Where do they all come from?

To find an answer, you could start in the garden of the Royal College of Physicians in Regent's Park. The college itself is one of London's most successful buildings. Designed by Sir Denys Lasdun and built in the mid-1960s, its flying Cubist forms, supported on thin columns and faced with off-white marble mosaic, offer an exciting counterpoint to the Nash terraces nearby. Inside it is full of space and light – and seems to be floating.

For decades it has had a medicinal garden which extends along the front of the college under the huge London plane trees and round to the rear; there, the sheltered lawn is dominated by a large plane, Platanus orientalis, reputed to be descended from the ancient tree on the island of Cos where Hippocrates is said to have taught.

But the gardens had fallen into disrepair until five years ago, when a decision was taken to re-plant them and appoint a garden fellow, Henry Oakeley, a retired physician and psychiatrist with a passion for botany. Under his direction, and with the expert input of gardeners Clare Beacham, Jane Knowles and Genevieve Newby, it has blossomed again.

There is a sense here, wandering among the beds in the company of Professor Oakeley, of returning to medicine's roots. One fifth of all the drugs we use today are derived from herbs. Many of the rest – antibiotics, for instance – come from fungi and moulds. But if you think Professor Oakeley is going to initiate you into the secrets of herbal medicine, be warned. While he is a true enthusiast for his subject and for the garden, of which he conducts regular public tours, he also has a healthy scientific scepticism for any claim that cannot be substantiated.

It turns out that many herbal remedies from the past, including some supposedly well established, were no better than placebos. Sixteenth-century herbalists judged the efficacy of a plant from its appearance, on the basis that God would not have created it to look as it did if he had not intended its form to act as a sign to its function. From this "Doctrine of Signatures", the mottled leaves of Pulmonaria, having an appearance like the cut surface of the lung, were prescribed for chest disorders. Hence its evocative name.

The beautiful blue liverwort, Hepatica nobilis, which is at its best in spring, was once cultivated as a liver tonic on the basis of nothing more than the three-lobed form of its leaves. At the height of its popularity, 300,000 tons of liverwort were harvested annually to make liver tonics for people who felt "liverish". Yet we never hear the term "liverish" now. Why? "Because there was no such thing," says Professor Oakeley.

In a nearby bed, there is a clump of evening primrose, which has brilliant sunshine-yellow flowers in season, still prized for its oil which is used as a herbal "immune stimulant". Professor Oakeley turns to me in mock exam mode. "Now doctor," he says, "you tell me how you stimulate the immune system except by getting a cold or a boil. To call it an immune stimulant is rubbish. This is bad science."

There are other traps for the unwary. St John's Wort is well known as one of the few proven herbal remedies for mild to moderate depression. Did you know, however, that there are six types of St John's Wort and only one, Hypericum perforatum (with the small yellow flowers and perforated leaves), will supply the necessary boost to your mood.

"There is nothing wrong with herbal medicine. It is just that 90 per cent of it is mythology," says Professor Oakeley, adding: "Anyone who believes plants are harmless should try cutting a chilli in half and applying the cut end to their eye."

Among the 10 per cent of herbal remedies that are effective are some life-saving medications. Taxol, a cancer drug used to treat ovarian and advanced breast cancer, is derived from the bark of the Pacific yew. Were that its only source, the Pacific yew would quickly have become extinct. The bark of 300,000 trees is required just to supply the North American market.

The Pacific yew was saved when scientists found a way of obtaining Taxol from the needles of the more plentiful European yew. Until a few years ago, if you had a yew hedge, people would pay you to take the clippings away. Today, Taxol is synthesised from cell lines derived from the yew – sparing the trees.

Our tour continues with Professor Oakeley striding past the borage ("encourages lust, treated plague"), greater celandine ("used for jaundice") to the back of the college where he stops by a clump of tall stems with lush leaves topped by red blooms. "The world's most poisonous plant," he announces. "Tobacco."

For some reason, this plant has escaped the confines of the college's "poison garden", hidden at the back behind a padlocked gate. Here is hemlock, with its big feathery leaves like huge cow parsley, used to kill Socrates.

Here, too, is the caster oil plant from which ricin is made, the foxglove (the heart drug, digoxin, that is derived from it is deadly in high doses) and the opium poppy. Saffron is also growing well – in overdose it causes palpitations, paralysis and death.

But there is one plant that heads the poison league – Aconitum napellus, grown for its tall spikes of attractive blue flowers. Used as the instrument of euthanasia for the "demented" in ancient Greece, it is not to be trifled with. For a physician gardener such as Professor Oakeley, however, it exerts an irresistible fascination – it is both a beautiful addition to the garden and a reminder of the fragility of life.

The Royal College of Physicians is open 9am-5pm, Mon-Fri, by appointment; call 020-7034 4901, or visit rcplondon.ac.uk/medicinal-garden/Pages/Overview.aspx