Fake or substandard versions of medicines are often hidden in cargos taking circuitous routes to mask their country of origin as part of criminal activity worth billions, they add.
They put people at risk of harm from medical products that may contain too much, too little, or the wrong active ingredient and/or contain toxic ingredients, said Margaret Hamburg, head of the U.S. Food and Drug Administration (FDA).
Counterfeiting is growing in complexity, scale and geographic scope, she said in a speech to the annual ministerial meeting of the World Health Organization (WHO).
In wealthy countries, counterfeiting often involves expensive hormones, steroids and anti-cancer medicines and pharmaceuticals related to lifestyle, a WHO report said.
But in developing countries, especially Africa, counterfeit medicines are commonly available to treat life-threatening conditions such as malaria, tuberculosis and HIV/AIDS, it said.
Nigeria, referring to a case involving tainted teething syrup in February 2009, said the consequences were often deadly.
Only last year we lost 84 children in Nigeria due to fraudulent practices in some countries. It is lives we are talking about, Nigeria's delegate told the talks.
Margaret Chan, WHO director-general, said that illicit products had also increased the problem of drug resistance, including to vital anti-malarials and HIV/AIDS drugs.
For a patient, any medicine with compromised safety, efficacy or quality is dangerous, she said.
Major generic drug makers India and Brazil, backed by health activists, charge that concerns about counterfeit drugs are being hijacked by pharmaceutical companies keen to protect their patents against legitimate generic competitors.
What we object to is a group of private companies, with the help of the (WHO) secretariat, waging war in this organization against generic medicines, Brazil's ambassador Maria Nazareth Farani Azevedo said in a speech.
Chan said that her United Nations agency would not be drawn into policing intellectual property (IP). The role of the WHO should be concentrating on public health, not on law enforcement nor intellectual property enforcement.
Research and development-based pharmaceutical companies say that counterfeit medicines pose a threat to patients and they are not driven by commercial interest in fighting the scourge.
There were 1,693 known incidents of counterfeit medicines last year, a rise of 7 percent, according to the Geneva-based group whose members include Bristol-Myers Squibb, Roche, GlaxoSmithKline and Sanofi-Aventis.