Pocket bongs, digital scales, herb grinders and exotic shamanic plants. The shelves of King Bong in Bournemouth are packed with all the discerning drug user could possibly want. There are brightly packaged pills such as "Yellow Veg-E's", "vegetarian blissed-out dance capsules", and the citrus-flavoured Lime-Fantazias which promise "euphoric sensations" all night. And all of it entirely legal.

King Bong is one of some 200 "headshops" in the high street. Most major towns and cities now have a resident drug emporium, selling raver toys, cannabis accessories, and a selection of legal, mind-altering drugs. Along with several large-scale websites, they form a lucrative "legal highs" industry which markets exotic mind-changing plants and chemicals to a growing audience of drug users looking for alternatives to illegal substances.

Demand for such legal highs has never been higher. "Business is on the up," says King Bong's owner, Tony Rotherham, who has run headshops for 12 years. "We're getting all sorts in here. Hippies, clubbers, students, housewives, even slimmers looking for appetite suppressants."

It was the boom in sales of magic mushrooms that kickstarted the industry. A legal loophole led to a proliferation of vendors. The mushrooms' reliable and mostly benevolent psychedelic effects changed public perception of legal highs. "Mushrooms opened people's mind to the possibility you could go into a shop and get a legal high that had an effect," he says.

When the loophole closed in July, vendors filled the void. "We've got a lot of new good products coming in," Mr Rotherham says. Some 20 new, effective drugs have emerged in the last year.

A hotchpotch of shamanic plants, synthetic stimulants, and psychedelic cacti, most imported from the Netherlands, New Zealand and India, are repackaged and sold across the UK. An intoxicant plant from Thailand called Kratom is a big seller and dubbed the "herbal speedball" due its apparent euphoric effects. Also selling well is a new breed of ecstasy-like drugs, marketed as "p.e.p pills". They contain piperazines, stimulant chemicals from the same chemical family as Viagra.

Magic mushroom sellers have switched to selling another mushroom, not yet outlawed: the red-and-white spotted Fly Agaric toadstool, which contains the psychoactive chemicals muscimol and ibotenic acid which can trigger delirious, dream-like states.

Thanks to the effectiveness of these legal highs and the large customer base created by the mushroom boom, the trade is booming in shops and on the web, with shoppers exploiting secure credit card orders and 24-hour websites.

"Loads of people are getting into it," says Mark Evans, owner of EveryOneDoesIt.com, the UK's biggest online headshop. "At the click of a button you can have whatever you want next day, at your home or in your office."

His site boasts more than 5,000 products and many types of drugs: stimulant, visionary, relaxant, aphrodisiac. Customers can give star ratings and post Amazon-style reviews. The web ensures word of mouth spreads quickly. Highs that don't work or have negative side-effects quickly disappear from sale.

It is proving a lucrative international business. "We're selling to thousands of customers a week all over Europe and north America," Mr Evans says. "There's a lot of money in the industry right now." His company claims a £2m a year turnover, with the estimated worth of the UK industry put at £10m.

While the majority of the new drugs remain unscheduled under the Misuse of Drugs Act, retailers are careful not to encourage any illegal activity or directly promote products as drugs. Cannabis seeds are often sold as "souvenirs". Bottles of inhalant amyl nitrite or "poppers" are advertised as "room odourisers". Bongs (a water pipe used for smoking cannabis) are labelled for "legal smoking mixes only". However, some of the plant-based highs have a quasi-legal status because they contain naturally occurring illegal drugs. The San Pedro cactus contains the outlawed psychedelic substance mescaline. Magic mushrooms, too, contain a class A drug, psilocybin, and were banned.

Says Katy Swaine, head of legal services at drugs advice charity Release: "The circumstances in which it is illegal to possess or supply the plant are ambiguous." But the authorities seem unconcerned. "We've only had the police in here once in 12 years," says Mr Rotherham. "The local beat officer came in after someone from a local mental health institute bought some herbal highs. He asked us not to sell to anyone from there. So we didn't."

Retailers say legal highs are safer than illicit drugs, which are often either adulterated or dangerously powerful. In contrast, many legal highs have a history of human use dating back thousands of years. Plant preparations such as the psychedelic Amazonian brew, ayahuasca, or Yopo Seeds, which contain the hallucinogen DMT (dimethyltryptamine), are used as sacraments in some shamanic ceremonies, with vomiting and loss of bowel control common.

Some have moved to ban some legal highs as their use has increased, creating a patchwork of inconsistent legislation. The potent psychedelic herb, Salvia Divinorum is banned in Australia and Italy but legal elsewhere.

"Governments tend to respond to very visible problems or public health scares," says Ms Swaine. "If there was an explosion in the use or supply of a particular substance, as with magic mushroom, they might take steps."

Mark Evans, of EveryOneDoesIt.com, says: "The government know we're here. We pay 40% tax plus 1% for national insurance ... I'm sure they do very well out of the headshop industry."

New stimulants, old plants

Salvia divinorum Rare form of sage contains Salvinorin A, powerful psychoactive chemical.

Effects When smoked, its effects are instantaneous, powerful and strange.

Duration 5-10 minutes, with an hour-long tail-off.

Status Banned in Australia and Italy.

Kratom (Mitragyna speciosa) Native south-east Asian plant. When eaten, the powdered leaves cause an effect not dissimilar to a combination of heroin and cocaine.

Effects Waves of physical pleasure, euphoria.

Duration 5-8 hours.

Status Illegal in Thailand and Australia

Fly Agaric (Amanita muscaria) Red and white spotted mushroom. When cooked the toxins break down, so it can be drunk in a tea.

Effects Often unreliable, a dreamy intoxicated state, excessive salivation.

Duration 5 hours

Status Legal everywhere.

San Pedro (Trichocereus pachanoi) Common cactus found in most garden centres, has large amounts of mescaline, class-A psychedelic.

Effects Mild stimulant or full-blown psychedelia for 12 hours. Large amounts of the bitter cactus must be consumed. Most throw up.

Duration 12-18 hours.

Status Legal.