GlaxoSmithKline is recalling all supplies of its non-prescription weight-loss drug Alli in the US and Puerto Rico after customers reported finding other pills and tablets in some bottles.

The news is a fresh blow for a product once touted as a potential blockbuster but which has had disappointing sales over the years - aggravated, in part, by a separate supply problem two years ago.

The British group said it believed that some US bottles of Alli might not contain authentic product, adding that it was working with the US Food and Drug Administration on the retailer-level recall.

News that bottles had been tampered with first emerged on Wednesday.

GSK has received inquiries from consumers in Alabama, Florida, Louisiana, Mississippi, New York, North Carolina and Texas about 20 bottles containing tablets and capsules that were not Alli.

The treatment is also sold in Europe but the company said no problems had been detected there and European sales were not affected by the US recall.

GSK said it was not immediately clear how the bottles had been tampered with or where in the supply chain the problem had occurred.

NO THEORIES YET

"The investigation is ongoing. We've asked people to return bottles to us, so we can examine them very closely ... we don't have any theories at this point," a company spokeswoman said.

Shoppers said they had found a range of tablets and capsules of various shapes and colours in purchased Alli containers. The authentic drug is a turquoise-blue capsule. Some bottles inside the outer carton were also missing labels and had tamper-evident seals that were not genuine.

There have been no reports of any serious illnesses related to the product after news of the tampering. - Reuters