The growing threat posed by dangerous fake drugs has been highlighted by a leading medical journal.

Between 2000 and 2006 the US Food and Drug Administration (FDA) saw an eight-fold increase in the number of new counterfeit cases, said The Lancet.

In developing countries with weak regulatory systems, around 10% - 30% of medicines might be counterfeit, the journal added.

Antimalarial drugs were a particular target for counterfeiters and fake drugs had flooded the market in many Asian countries.

Worldwide sales of counterfeit drugs were forecast to reach 75 billion US dollars (£38.22 billion) in 2010.

Counterfeit medicines are defined by the World Health Organisation (WHO) as drugs that have been "deliberately and fraudulently mislabelled with respect to identity and/or source".

Counterfeiting may have caused the deaths of at least 81 patients in the US who died after being treated with contaminated heparin, a widely used blood thinning drug.

Last week the FDA told a Congressional hearing it believed a dangerous contaminant found in batches of the heparin may have been deliberately added. The contaminant, traced back to a Chinese supplier, was structurally similar to heparin but 100 times cheaper.

Substances used to adulterate medicines varied from chalk to antibiotics to highly lethal substances, said the editorial.

The counterfeit drug trade was becoming more difficult to combat, it pointed out. Criminals were using more sophisticated techniques to bypass standard laboratory tests, for instance by adding cheaper substances that mimicked genuine drugs.