Some of the health benefits claimed for a new weight loss drug may not be justified, say experts.
The drug works by reducing appetite
Rimonabant, launched in the UK last summer, has been shown to aid weight loss by reducing appetite.
But a Drug and Therapeutics Bulletin paper suggests claims that it also has an additional positive impact on the body's chemistry have not been proved.
However, the manufacturers said the findings had proved consistent across all trials.
The National Institute for Health and Clinical Excellence (Nice) is currently appraising the drug for use on the NHS.
Manufacturers Sanofi-Aventis claim it has been shown to cut levels of potentially harmful cholesterol, fats and sugars in the blood to a greater extent than would be expected by weight loss alone.
In theory, this should help to reduce the risk of developing type 2 diabetes and heart disease.
But the DTB paper argued that research had failed to prove that any positive impact on body chemistry was solely down to taking the drug.
It was possible, for instance, that it was down to advice given to patients taking the drug to lead a more healthy lifestyle, and take more exercise.
The paper also highlighted the fact that in trials rimonabant had no effect on levels of "bad" cholesterol, and little or no effect on blood pressure.
It said the drug had not been effectively compared with other, cheaper weight loss drugs, such as Xenical (orlistat) and Reductil (sibutramine), which are both approved for NHS use.
COST OF WEIGHT LOSS DRUGS
Rimonabant: £720 a year
Orlistat: £520 a year
Sibutramine: £480-£570 a year
The DTB paper stated: "Orlistat is the drug for obesity for which there is the most evidence for efficacy and safety to date, and we have previously concluded that it is a reasonable option for obese patients where diet and exercise and/or behavioural measures alone have failed."
However, Dr Ian Campbell, medical director of the charity Weight Concern, said research did suggest that rimonabant had an extra effect on body chemistry over and above that expected through losing weight alone.
He said this might be a direct result of the unusual way it works on fat cells.
Dr Campbell said: "It's a new drug and we need more time to become fully aware of all its effects.
"It is more expensive than other available drugs but should be considered when the benefits of weight loss for the patient can justify the investment."
A spokeswoman for Sanofi-Aventis said the effects on body chemistry had been consistently seen in all the trials of rimonabant.
Further trials were underway to examine the effects of the drug further.
She said there was no doubt that adopting a healthier lifestyle could have a positive impact, but in trials people given a dummy drug also improved their general lifestyle without the same level of effect seen in those taking rimonabant.
The Medicines and Healthcare products Regulatory Agency (MHRA) rejected a complaint last year that an advert for rimonabant had exaggerated its benefits.
A spokesman for the MHRA said it would examine the latest claims.