Many bogus medicines have no active ingredient at all while others contain the wrong dosage or are manufactured in such a way as to be ineffective.

Harvey Bale, Director General of the International Federation of Pharmaceutical Manufacturers Associations, told reporters the illegal trade in counterfeit medicines affected both branded and generic medicines.

In Nigeria, around half of all medicines on the market are estimated to be counterfeits while nearly 40 percent of anti-malaria pills sampled in Southeast Asia did not contain any active drug, he said.

So far, Western countries have largely escaped the problem but there is a growing danger of counterfeits entering Europe and the United States from North Africa and Mexico.

"The priority of police forces round the world is to focus on illegal drugs, like heroin, not on counterfeit versions of legal drugs," Bale said.

"Unless we begin to tackle the problem in developing countries, the problem won't stay there."

Several companies have issued warnings in recent months about counterfeit versions of their medicines reaching the U.S. market. They include Johnson & Johnson (NYSE:JNJ - News), Amgen Inc (NasdaqNM:AMGN - News), Serono Inc (SEOZ.VX), GlaxoSmithKline Plc (London:GSK.L - News) and Eli Lilly and Co (NYSE:LLY - News).

Criminal gangs are forging a range of prescriptions products -- and in many cases it is far from easy to spot the counterfeit. Bale said fake versions of the powerful anti-AIDS drug AZT, made by Glaxo, had been detected in Asia containing the correct chemical ingredients but the wrong coating, so the medicine passes through the body without any effect.

The most counterfeited drug in the world is an inexpensive antibiotic derived from penicillin, called amoxycillin, he added.

Within Europe, Russia has emerged as a hotbed of counterfeits. Russian officials had detected only one imitation brand in 1997 -- today, the number has risen to more than 170.