'I thought it was healthy to see a doctor to lose weight,' she says.

Ng says the doctor didn't tell her what was in her pills, but odd things happened after she took them.

'I ended up losing lots of hair and it was so scary that I stopped immediately. I lost all my confidence [in the doctor],' she says.

However, Ng was undaunted. She sought weight-reducing drugs on the internet, because 'those off-the-shelf drugs are too safe and too mild'. And on the strength of some 'trustworthy' online reviews, Ng bought several products purported to be 'all-natural Chinese herbal medicines with no side effects'. They helped her lose weight but, again, at a cost to her health.

'I stopped every time the side effects proved too severe,' Ng says. 'What's the point of losing weight if I can't look healthy? But I still kept trying new ones as I thought I could find one that would work for me.'

Despite repeated government warnings on the health risks of slimming drugs, more women are reporting adverse reactions from consuming potent treatments purchased from pharmacies, clinics and the internet.

As of last month, the Department of Health had been informed of 18 cases this year of women falling ill after consuming slimming products with banned substances, up from six last year. A 13-year-old girl who had ordered drugs on the internet was admitted to hospital in June with symptoms including palpitations, insomnia, sweating and tremors, the department says.

The department has found that a number of off-the-shelf slimming products contain prescription-only appetite suppressants such as sibutramine and phentermine - substances notorious for unpleasant side effects ranging from insomnia and depression to constipation and hypertension.

From 2005 to July this year, 44 of 935 slimming products tested were found to contain such 'undeclared drug ingredients', and half of them were bought on the Web. The department says it will now increase regular random checks on products offered online.

Since 2005 it has made 50 calls for websites selling items with undeclared substances to remove those product listings from their pages. Yet online slimming pills are still increasingly popular with women who need a last-resort quick fix, experts say.

'Those who have tried everything to no avail might look at these drugs and think to themselves, 'Why not give it a try, because it's so readily available?',' says William Chui Chun-ming, education director at the Society of Hospital Pharmacists.

Yahoo lists hundreds of slimming products, each promising results that seem too good to be true. Bodycode's Empress Sorceress Soup says it can help a patient lose 4.5kg in a week, while others such as Edema Tea claim to prevent cardiac disease and puffy eyes. Beauty forums on sites such as Uwants.com and She.com are often packed with discussions and testimonials about slimming products.

Chui urges extreme caution in taking slimming pills. 'The place of origin is often unclear and the products are more likely to be counterfeit with impurities and no quality control,' he says.

So-called 'all-natural' herbal medicines could also be mixed with potentially dangerous ingredients, says Fu Wenshu, a Chinese medicine consultant at the Chinese University.

'No western medicine should be taken two hours before and after Chinese medicines, and definitely not together. It can lead to poisoning,' Fu says.

'There are many kinds of obesity. It can be caused by an illness or emotional distress. A drug that induces diarrhoea would be dangerous for a patient who suffers from dehydration. People who take drugs regardless of their health are putting their lives at risk, and it's a waste of time and money.

'Slimming is an income-earner for the beauty industry and using drugs meant for treating clinical obesity in that capacity is grossly inappropriate and irresponsible,' she says.

Despite the notoriety of slimming drugs, many people still hope to lose weight without controlling their diet and exercising. And although unethical private doctors, slimming companies and negligent apothecaries are complicit in making the drugs easily available, the biggest problem lies in attitudes towards slimming.

'You can blame all the usual suspects for giving people access to these potentially dangerous drugs and tighten control on this part of the problem, but the public needs to wake up to the fact that popping a pill is not the answer to losing weight,' says Emily Chan Ying-yang, associate professor at the Chinese University's School of Public Health, citing recent government infomercials as the first step in the right direction. 'We must educate the public to make informed and intelligent decisions for themselves,' she says. 'We can't stress this enough. Medication is always the last resort, and there's no fast track to losing weight - it must involve lifestyle changes.'

Kennis Woo Ka-lai feels much better since she swapped slimming pills for a weight regimen supervised by a nutritionist. She says she was kept in the dark about the drugs a doctor prescribed her for two years and couldn't account for her mood swings at the time.

'Now I think it's very probably the drugs that caused it,' she says.

Woo gave up the pills, fearing that they might increase her risk of a miscarriage.

'People take those pills because they think it's their last hope,' says the 25-year-old. 'I used to exercise and diet, but I guess I wasn't doing it properly. It's good and motivational to get professional help and someone to watch my progress.'

'Most people who lose weight rapidly with pills rebound to their original weight or get even heavier than before the treatment,' says Lee Siu-suet, a nutritionist at the Chinese University's Centre for Nutritional Studies. 'They come to our centre to seek healthier and longer-lasting weight-loss methods.'

Yet after all her bad experiences with slimming products, Ng hasn't given up on her quest for a slimmer figure. She still wants to lose another 5kg - even though at 1.64 metres tall she weighs just 52kg - and is investing in an expensive health supplement from a controversial brand based in Los Angeles.

'I like to think I am smarter than before,' she says. 'It looks like a quality product, so I think it's different. At least it makes me feel better.'