It is not uncommon for these drugs to be found in slimming supplements in Hong Kong. Last year, the Department of Health found 46 such products contained western pharmaceutical ingredients. The year before, there were 43 cases.

While western drugs are strictly controlled in Hong Kong, health supplements are not subject to regulation. Even if the authorities discover adulterated health products, the lack of a registration system means the chances of tracing the manufacturer or distributor are slim.

It is a situation the medical and pharmaceutical professions have urged the government to address, and one which has taken on a more acute sense of urgency after a recent discovery at the Hospital Authority's Toxicology Reference Laboratory.

Dr Tony Mak Wing-lai and his colleagues at the laboratory have found slimming products that contain 'analogues', which are created by modifying the chemical structure of pharmaceuticals.

By altering the structure of drugs slightly, you are likely to change their effect and level of safety. Unlike the pharmaceutical they resemble, analogues have not been tested for efficacy and safety, a process which usually takes more than nine years and costs up to US$802 million.

Analogues are also able to escape detection by health regulators. As Dr Mak, consultant pathologist at the laboratory, said: 'There's a list of things they [health authorities] need to test for - 20 well-known regular slimming agents.

'If it's not on the list, you will miss it, and these analogues are much more difficult to find. Unless you are testing for it, you won't find it.' His findings, documented in last month's edition of the Hong Kong Medical Journal, came as a result of two years of testing for adulterated slimming products. Between September 2004 and December 2006, the lab studied 42 patients with clinical problems related to the use of slimming products.

Four cases revealed the use of analogues in the products. In one case, a 33-year-old woman required a liver transplant after using a slimming product which purported to use 12 herbal ingredients.

Another woman, aged 53, who took a slimming product containing an analogue was not as fortunate. She was admitted to an accident and emergency department in June 2005 suffering sudden cardiac arrest and died four days after admission.

Two of the slimming products contained N-nitrosofenfluramine, an analogue of the banned fenfluramine drug. The other contained N-desmethyl-sibutramine, an analogue of sibutramine.

Although only four people suffered severe side effects, there could be hundreds taking the product. 'The number of cases is not important,' Dr Mak said. 'One single incident is already alarming. I can't tell you where it comes from. Some people are doing this illegally. It's for the government to find out. How to control the source is one thing, but at least the government should make sure these products on sale are safe and not contain this kind of analogue. My concern is how to eliminate it.'

Dr Mak wants legislation to govern the sale and distribution of slimming products. Most cases of adulterated health products are discovered as a result of people suffering adverse effects, rather than through random testing.

The Department of Health says it takes samples of health products from various market outlets to test for the presence of any western drug ingredient.

'These samples are taken either during routine surveillance or upon the receipt of complaints. If a product is found to contain a western drug ingredient, the importer or wholesaler will be instructed to recall the product from the market,' a spokeswoman said. 'The department will also collect evidence and initiate prosecution against the importer or wholesaler if there is sufficient evidence.'

The government has been lobbied in the past for laws to be drafted controlling the sale of health products, but with mixed results.

Benjamin Kwong Yiu-sum, president of the Pharmaceutical Society of Hong Kong, said part of the problem was that the government found it difficult to decide which health products required regulation. 'Do we regulate vegetables and fish? Of course not,' he said. 'They [the government] really don't understand the concept of health food.'

The society believes any products which purport to boost health or body functions should be regulated.

'For any product, if they market or sell it to the public in the name of getting you better or improving any kind of performance, modifying body shape ... it should be regulated,' Mr Kwong said.

Although the products come from across the globe, on the mainland the chemical synthesis industry is booming. Mr Kwong said it was easy for the manufacturers of some slimming products to get their hands on western drugs, or similar versions of them.

At the least, regulations could involve a registration system, he said. This would involve taking

the name and address of the manufacturer, distributor and importer. 'If anything happens with the product, the government should know who to go to.'

Legislator Kwok Ka-ki, who represents the medical profession, believes one of the biggest hurdles

to regulation of these products is politics.

The closest Hong Kong came to regulation was in 2006, when the Undesirable Medical Advertisements Ordinance was passed, narrowing the claims that could be made in relation to health products. But the law was watered down - a proposal to curb the claims made by manufacturers of slimming products and those which affected the immune system was dropped after lobbying by the health supplements industry.

'By and large, most if not all the health products belong to those categories,' Dr Kwok said. 'I was very disappointed they were deleted. At the end of the day, the legislation is far from any effective control.'

When the bill was being debated, it was estimated the health supplements industry was a HK$2 billion business - and that putting restrictions on the products' claims would lead to job losses.

Dr Kwok has asked the government for a review of the law. He also wants a registration system. 'If western drugs are found in slimming products, the Department of Health can ban them - but because there's no legislation or registration, they simply don't know where the products are coming from,' he said. 'How do you get the people behind it? How do you get the distributor to court? Sometimes they get the distributor to court, but if they don't know them, they can't charge them.'

Other countries control health supplements through registration, or insist the manufacturers prove the claims of the product, as is the case in the European Union.

As Dr Kwok said, some of the products contained not just slimming drugs but other western pharmaceuticals with side effects that would give a 'slimming' effect, such as vomiting, diarrhoea and nausea.

Health supplements are widely used in Hong Kong: a household survey by the Census and Statistics department found in late 2005 that 22.8 per cent of people aged 15 and over had taken health supplements in the previous 12 months.

And 80 per cent did so to improve their body functions, while 26 per cent did so to prevent disease.

Ms Lee of the Centre for Nutritional Studies said about one in five of her patients used a slimming product, the youngest being just 16.

Philippa Yu, executive officer of the Hong Kong Eating Disorders Association, cited a 2003 study in which 600 local women were asked what methods they used to shed weight. Slimming products were the most popular option with 467 using them to lose weight.

'I think they are attentive to the slimming advertisements saying how much and how fast they can have weight loss and I think they underestimate the side effects,' Ms Yu said.

Some will stop taking the products when they suffer an adverse effect, but will just move on to another. Many are buying the products online. 'It's time for the government to strengthen the legislation and monitoring,' Ms Yu said.

She said some women reported side effects such as insomnia, loss of appetite, hand trembling and an accelerated heart rate after taking slimming products. 'They want a quick-fix solution just because they have a banquet to go to next month ... and it's a false hope.'