At first glance cannabis appears an unlikely candidate in the search for a weight-loss miracle drug, but the very same compound which switches off the same brain circuits that make people hungry when they smoke cannabis, could may well be the first blockbuster anti-obesity drug ever.

Analysts are apparently already predicting sales of over $3 billion a year.

Acomplia, or Rimonabant, produced by drug company Sanofi-Aventis could be granted approval by U.S. regulators as soon as next month.

Two other experimental drugs from Arena Pharmaceuticals and Alizyme, which use different methods of action have also produced promising results in clinical trials recently.

All three are part of a new wave of treatments and may mean even bigger profits for pharmaceutical companies.

It is however a high risk area, as slimming pills,historically have a dubious past, mainly due to ineffectiveness and adverse side effects.

Probably one of the most notorious is the diet drug combo "fen-phen," which was linked to heart-valve problems and has cost drug company Wyeth more than $21 billion in litigatious patient claims.

According to analysts there are now 26 new drugs in clinical trials for obesity and a further 32 in the pipeline.

This is along with more than half a dozen diabetes medicines currently being trialed as treatments for weight reduction.

According to World Health Organization (WHO) figures more than 1 billion people in the world are overweight and, if the current trends continues, that number will reach 1.5 billion by the year 2015.

More than 300 million apparently already rank as obese, putting them at substantial risk of heart disease, stroke, type 2 diabetes, respiratory problems and some cancers.

The problem is not confined to developed countries as even in developing countries, including parts of Africa, obesity is on the rise, with the western Pacific islands of Nauru and Tonga currently holding the unenviable position of the highest percentage rates of obesity in the world.

The scale of the problem, indicates that the arrival of new weight-loss drugs cannot come quick enough and will be welcomed.

However in reality, as experts point out, medication will help only a very small minority of patients and will not have an impact on the obesity epidemic.

They say physical activity and diet are far more cost-effective.

However Professor Luc Van Gaal of Belgium's University Hospital Antwerp, the lead investigator for one of the main clinical studies for Acomplia, argues that doctors must accept that dieting and exercise alone often produce disappointing results, leaving patients at risk of falling seriously ill.

Drug therapy, he says is the answer for certain obese patients who are at risk.

There are currently two main obesity medicines on the market, Roche's Xenical, which works by limiting fat absorption, and Abbott's Meridia/Reductil, which suppresses the appetite.

The adverse side effects, have resulted in modest sales of both drugs; Xenical can cause excess gas and oily discharge, while Meridia may increase blood pressure.

GlaxoSmithKline too is hoping for approval from a U.S. panel later this month for its own low-dose, low-side effect version of Xenical to be sold without prescription, but many doubt it will become a major seller.

By contrast, the consensus forecast from analysts for peak sales of Acomplia is $3.1 billion, with some predictions topping $5 billion.

Just how big the drug Acomplia will be will depend on the terms of use that Sanofi agrees with healthcare regulators.

The company is hoping to prove that Acomplia specifically reduces key cardiovascular risk factors, as a simple obesity label may not be enough secure widespread reimbursement from governments and insurers.

With Acomplia too, side effects may be an issue as though patients are reported to have lost around 7 kg (15 lb) in body weight over two years, there are concerns regarding its potential impact on mood.

However the very fact that Sanofi's rivals Merck, Pfizer and Bristol-Myers Squibb are also developing similar cannabinoid receptor blocking drugs, means they think Sanofi is on to something.