I'm extremely cautious about recommending slimming pills or anti-obesity medication as they're more formally called. Very few people benefit from them in the long term, and they should be viewed generally as a last resort, to help people who are grossly overweight, have already lost some weight and are now struggling to lose more.

A history of side effects

Problems with slimming pills in the past have left doctors very guarded about their use. Amphetamine-type slimming pills were popular 30 to 40 years ago, but it took some time for doctors to realise they can be addictive and harmful. Despite these problems, they were still widely handed out, often by poorly trained medical staff. Few people managed to diet successfully with them and, more importantly, keep weight off and many became physically hooked or addicted to them.

Many slimming pills, including more recent and supposedly safer types of appetite-suppressant amphetamine drugs, such as dexfenfluramine, have now been taken off the market because of potential side effects and complications, such as heart disease.

New slimming drugs

However, as obesity is such a current concern, there's a huge effort going into developing effective and safe medicines to help people lose weight and in recent years some of the products of this research have come into use.

Current anti-obesity drugs can be broadly divided into three categories:

• Drugs which increase energy expenditure. The idea of medicines which help us to burn up energy is very attractive, but it has been hard for researchers to safely put this into practice. Scientists got very excited about the idea of switching on our brown fat - a special type of fat that responds to nerve signals by using up more energy. But while the concept worked in laboratory rats, it hasn’t been safe or effective in humans.
• Drugs which decrease energy intake (appetite suppressants). This is the main focus of research because taking in more energy-rich food than is needed is the main problem behind being overweight. New appetite suppressants now in use include sibutramine and rimonobant. Sibutramine, for example, alters the chemical messages in the brain that control how the person feels about food. By influencing brain chemicals called noradrenaline and serotonin, sibutramine helps to make a person feel full. These drugs, when used with a weight-reducing diet and exercise, help people to lose more weight than a weight-reducing diet and exercise alone. But neither drugs work on their own - you can't carry on with unhealthy eating and still lose weight as it doesn't make you lose weight, just helps you to diet.
• Drugs which inhibit the digestion and absorption of food. One example of these drugs is Orlistat which inhibits the action of enzymes in the gut called lipases that are responsible for the digestion and absorption of fat in the food we eat. According to The National Institute for Clinical Excellence (NICE) average weight loss with Orlistat is only about 2 to 5 kilograms per year above placebo treatments (as well as small but significant reductions in cholesterol levels and blood pressure, and a delay in the onset of diabetes in those at risk).

Other anti-obesity drugs are currently in development and may come into general use in the next few years.

The Royal College of Physicians (RCP) advises doctors to only prescribe anti-obesity drugs in special circumstances. For example, as part of an overall treatment plan for the management of obesity for people aged 18 to 65 years with a body mass index (BMI) of 30 or more. Or where someone has a BMI of 27 or more and has another significant disease, such as type 2 diabetes or high cholesterol. The person must have already made serious attempts to lose weight by dieting, exercise and/or other changes in their behaviour but failed to lose 10 per cent of their weight after at least three months.

It's important to be aware that not everyone responds to drug therapy, and most doctors recommend that weight loss should be measured after 12 weeks and the anti-obesity drugs stopped if someone has not achieved a 5 per cent weight reduction. And, like any treatment, they have side effects. Sibutramine sometimes increases blood pressure, for example, while Orlistat can cause diarrhoea and flatulence.

Slow metabolism is a myth

In general, the idea that overweight people have a slow metabolism is a complete myth. The heavier you are the harder your body has to work to carry the weight and the greater your metabolic rate. The only exceptions are people with conditions such as a clinically under-active thyroid gland, who are generally ill with other symptoms too.

Healthy eating

By far the healthiest and most effective way to control your weight is through long-term healthy eating and regular exercise (you will need to do at least 150 minutes a week of seriously-out-of-puff exercise, ideally broken down into daily chunks) supported by counselling and behaviour modification, which you should find at a slimming club.

This article was last medically reviewed by Dr Trisha Macnair in July 2008