They've been approved for use only since 1997 in the US, and a year later than that in Europe. Yet, they are already achieving sales of US $500m every year, a figure that is expected to treble by 2010.
This is an extraordinary success story for a class of drugs that doesn't work, that hasn't been properly tested and may cause serious side-effects.
The two drugs currently on the market-orlistat (Xenical) and sibu-tramine (Meridia)-fail to reduce weight to even the minimum stand-ards recommended by health groups. The US National Institutes of Health (NIH) has said that just a 10-per-cent loss in weight for anyone with a body mass index (BMI) of 30 or more is enough to reduce the threat of type 2 (non-insulin-dependent) diabetes and heart disease.
Europe's drugs regulator, the European Agency for the Evaluation of Medicinal Products (EMEA), has accepted this measure, and has declared that every anti-obesity drug should, at the very least, achieve this target.
But sadly, this requirement was not included in any licensing protocols. If it had been, then neither orlistat nor sibutramine would have been approved in the EU countries. Across limited studies, the drugs have achieved weight losses of only 5 per cent-while the 10-per-cent optimal figure has been achieved only when the patient has also introduced life-style and dietary changes.
If this is the case, it suggests that doctors are wrong in prescribing the drugs in the first place. Among the strict requirements of prescribing is that these agents should only be given to obese people who are unable to lose weight by simply changing their lifestyles. As such, they are seen as a last resort for those who either lacked the willpower to change their diet and exercise habits, or whose weight did not respond when these changes were made.
In one four-year study of 3305 obese patients, orlistat achieved an overall weight loss of just 2.9 per cent, and only 43 per cent of the participants completed the trial (Diabetes Care, 2004; 27: 155-61).
Sibutramine fared only slightly better. In three weight-loss trials, each one lasting a year, the drug reduced the weight of participants by an average of 4.8 per cent, and the dropout rates averaged 48 per cent (J Obes Relat Metab Disord, 2003; 27: 1437-46).
The loss of study participants in these obesity trials has been so marked that researchers at the University of Alberta Hospital, in Edmonton, Canada, say that the results are virtually meaningless (Lancet, 2007; 369: 71-7).
But if the incomplete trials have failed to prove the drugs' effective-ness, they have uncovered a range of adverse effects. Gastrointestinal problems are common among people who take orlistat. In one study, up to 30 per cent of patients experienc-ed increased defecation, while 7 per cent suffered from fecal incontinence (Cochrane Database Syst Rev, 2003; 4: CD004094).
Adverse reactions to sibutramine are more wide-ranging, and are typical for an antidepressant, which is what the drug was originally designed to be. Common side-effects include insomnia, nausea, dry mouth and constipation (Obes Res, 1999; 7: 363-9).
In Italy in 2002, the drug was temporarily taken off the market after it was suspected of killing two patients. It increases blood pressure and pulse rate, and the Italian reports suggested that it also causes tachycardia (increased pulse rate) and an irregular heartbeat (arrhyth-mia).
However, as the Italian regulators were unable to prove conclusively that the drug was responsible for the deaths, it was reintroduced. Never-theless, sibutramine is not recom-mended for patients with uncontrol-led hypertension, preexisting heart problems or tachycardia (Drug Safety, 2003; 26: 1027-48).
And now, a new anti-obesity drug is about to join this lucrative market. Rimonabant has already been approved by the EMEA, and the US drugs regulator, the Food and Drug Administration (FDA), is currently reviewing it.
It's a new type of drug known as a CB1-selective ligand, and its phar-macological base is Cannabis sativa, the mood-altering plant that's been used recreationally for hundreds of years. The drug was initially develop-ed as both an anti-obesity and a stop-smoking aid, but the latter program was abandoned.
The drug has been tested in a series of trials known collectively as the Rimonabant In Obesity Program, or RIO. At the one-year stage, the drug did not work at the 5-mg dose, but 20 mg was found to be marginally more effective than either orlistat or sibutramine, achieving a weight loss of 5 per cent (Cochrane Database Syst Rev, 2006; 4: CD006162).
Common adverse reactions are nausea, dizziness, diarrhoea and insomnia, affecting up to 9 per cent of users. However, while the 20-mg dose can help weight loss, it is also less well tolerated, causing up to 16 per cent of participants taking this dosage to leave the trial prematurely (N Engl J Med, 2005; 353: 2121-34).
In three of the RIO studies, 7 per cent of participants had to drop out because of psychiatric disorders, with most reporting constant bouts of depression. Researchers have estimated a 5-per-cent overall risk of depression with rimonabant, although the Canadian researchers believe this is a conservative figure as people known to have mental illness were excluded at the outset.
But perhaps the most worrying aspect of the anti-obesity drugs is their long-term effects. As the drugs are still relatively new, and because virtually no trial has so far been successfully concluded, no one has any real idea as to what is happening to the people taking these drugs.
The one certainty is the enormous profits they are bringing to the drug companies.