Health experts have questioned the health benefits of a new weight loss pill. Rimonabant was launched last summer amid claims that it could cut levels of potentially harmful cholesterol, fats and sugars in the blood to a greater extent than would be expected by weight loss alone. However, now health experts have said that the manufacturer's claims that it has an additional positive impact on the body's chemistry have not been proved.

The National Institute for Health and Clinical Excellence (Nice) is currently appraising the drug for use on the NHS following the claims by manufacturers Sanofi-Aventis regarding its health benefits. They claim that the benefits listed above should help to reduce the risk of developing type 2 diabetes and heart disease, as well as helping many overweight people to lose weight.

But a Drug and Therapeutics Bulletin paper argues that research had failed to prove that any positive impact on body chemistry was solely down to taking the drug. They say possible health improvements could be down to advice given to patients taking the drug to lead a more healthy lifestyle, and take more exercise.

The paper also stated that in trials of rimonabant it had had no effect on levels of "bad" cholesterol, and little or no effect on blood pressure. It said that 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.

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. 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 and that 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 but said it would look into the new claims.