Long dismissed as unscientific, plants are making a pharmaceutical comeback, says Roger Dobson
Flowers - the lover's gift, the delight of an English garden - are more than they seem. They can sting, poison, cause addiction and allergy, and sometimes they can kill. Even plant-loving visitors to the Chelsea Flower Show this week are being warned about those that can harm them.

But the power of flowers is also being harnessed by teams of researchers around the world looking for cures and treatments for a huge range of diseases and disorders.

A report from the Royal Botanic Gardens at Kew suggests that over the past 10 years the number of companies carrying out research on plants has increased from zero to 223. The renewed interest in plants as reservoirs of valuable medical compounds has resulted in record levels of funding being allocated to researchers. One research institute in Costa Rica has been given $1m (pounds m) to screen micro-organisms for biological activity.

The revival of interest in ancient herbalism means that plants are currently being investigated for use in the treatment of HIV, Aids, cancers, dementia, heart disease, liver disorders, chronic pain and leukaemia. The unlikely bouquet includes daffodil bulbs processed for a new treatment for Alzheimer's sufferers and marigolds as an anti-inflammatory drug. Ginger is also on trial as a treatment for morning sickness in pregnancy, and several teams are working on gingko for the treatment of asthma. Also under active investigation are bluebells, snowdrops, hyacinths, daisies, mistletoe, mulberry, harebells, thistles, garlic and yew - and cannabis.

This is what Professor Fred Evans's patients are being given at the Hammersmith Hospital, London, to get rid of chronic nerve pain. It is the first time that cannabis has been used in the NHS for 40 years and tests on animals by Professor Evans and his team indicate that it is 357 times more effective than aspirin in controlling pain in a peripheral nerve damage as a result of viral infections.

Professor Evans, head of London's Univeristy department of Pharmacognosy, who is carrying out the trial with a consultant anaesthetist, Dr Anita Holdcroft, said: "We are using cannabis in a clinical trial on patients experiencing pain from peripheral nerve damage as a result of viral disease. The patients we are looking at have tried all the available analgesics and could no longer tolerate these drugs because of their side- effects. We did need to get a Home Office licence to work on the drug and it is the first British trial for 40 years.

"One of the things we hope to show is that cannabis does not have to produce a high to give pain relief."

Across London at Guys, another plant, garlic, is being used in clinical trials to treat heart disease and for lowering cholesterol levels. Meanwhile, researchers at Kew Gardens have been working with Charing Cross Hospital on the use of mulberry roots in the treatment of diabetes.

In HIV and Aids research, a range of plant compounds are being developed which interfere with replication of the virus. The most promising so far are compounds from an Australian tree, the Moreton Bay chestnut, and much of the original research has been carried out at the biological interaction department at Kew.

Dr Monique Simmonds, head of the department, said: "We have been working on a number of plants. We are at the early stages in several areas, but a lot of things look promising. We are looking at novel compounds and continuing our work on the HIV looking for related compounds."

Anti-cancer components have also been found in the yew and mistletoe, as well as from marine sponges.

In Aberystwyth, Dr Robert Nash and a team of researchers have been testing the bluebell and have discovered a number of key elements that may be useful in the treatment of HIV and cancer.

This team is also working on a plant they have found to have anti-leukaemia effects. The name of the plant is being kept secret pending a patent. Dr Nash, a plant chemist at the Institute of Grassland and Environmental Research at Aberystwyth, said: "Traditional herbal remedies are providing useful indicators to which species may be worth studying for compounds of potential pharmaceutical activity, and we have already found one compound with anti-leukaemia effects."

This secret compound is being evaluated at the National Cancer Institute in the United States. Dr Nash and his fellow researchers have also been looking at a number of plants, including the bluebell: "We began by looking at the bluebell to see why it was toxic to cattle and horses, and when we analysed it we found 12 new alkaloids that are related to some from tropical plants which have been shown to have anti-HIV and anti-cancer activity. The hyacinth has been found to have a compound which has been synthesised as a treatment for diabetes," he said.

Around 300 years ago the medical profession became more scientific, more chemically based, and botany and plants were increasingly marginalised as being out of date by the medical modernists. Dr Liz Williamson, a lecturer at London University's Department of Pharmacognosy, said: "In the West, I think we let synthetic chemistry take over, and we ignored the plants that had been used for centuries. The fashion was to make drugs, when many were naturally occurring under our noses. We must not forget that 80 per cent of the world relies on plant-based medicines, and I think the increasing activity here is because there is a growing awareness of the value of what is out there.

"We have always known some of the plants have medicinal effects, but have never been able to isolate them," she added. "It was a bit like Leonardo inventing the helicopter but not having an engine to fly it. We have now developed more sophisticated techniques to find these things."

The fact that big money has been made from three anti-cancer drugs derived from plants - etoposide, vincristine and vinblastine - may also account for the new interest in plants. Vincristine, and vinblastine are drugs derived from the Madagascar periwinkle, and have added considerably to the income of Eli Lilly. Etoposide comes from the roots of the north american may apple. Taxol, from yew trees, currently on trial in the treatment of breast cancer, also looks destined to be a successful drug.

In some cases plants are being specifically grown for pharmaceutical research. In Lincolnshire, Bob Out is growing daffodil bulbs, which are shipped to a pharmaceutical company in Edinburgh where the compound galanthamine - also found in snowdrops - is extracted. Research shows that the compound, a self-defence product which the plant uses to cause nausea in animals who eat it before the flower sprouts, may slow down the progression of Alzheimer's disease.

"To meet our target the aim is to produce 20,000 tonnes a year, and we hope to do that over the next few years," said Mr Out, managing director of Lingarden, the world's biggest producer of daffodils.

Some of the compounds isolated will be used as they are, or in concentrated form. Some will be synthesised and mass produced by pharmaceutical companies. One of the problems the companies face is that they cannot patent plants, only the specific techniques they use in manufacturing.

Enthusiasts such as Dr Williamson believe there is enormous scope for research into plants as natural pharmaceutical factories. In Aberystwyth, Dr Nash agreed: "Everything is there to be looked at. Nothing should be ignored, not even the potato. We thought everything was known about the potato until we analysed it and found a new group of compounds that had been missed."

And the good news for researchers is that there are years of work ahead - 95 per cent of the world's plants have not even been looked at yet.