The world-wide use of fake drugs has increased because they are so easy to make and sell cheaply, says the World Health Organization.
Not all pills can be relied upon
It is launching a campaign to clamp down on the use of the drugs, which it warns can be harmful and even deadly.
The WHO estimates up to 25% of medicines consumed in developing nations are counterfeit or substandard.
They are often used to treat life-threatening conditions such as malaria, TB and Aids.
The problem is also widespread in richer countries, according to the WHO.
One of the best selling fakes is Viagra, which can easily be bought on
the internet, it warned.
WHO director general Dr Lee Jong-wook said: "Combating low quality or illegal medicines is now more important then ever.
"Expanding access to safe, effective treatment for AIDS and other illnesses is no longer an option, it is an imperative."
The US Food and Drug Administration estimated that fake drugs
alone comprise more than 10% of the global medicine market - generating annual sales of more than $32bn.
A WHO survey between January 1999 and October 2000 found that 60% of fake medicine cases occurred in developing countries and 40% in industrialised nations.
Daniela Bagozzi, a WHO spokeswoman, said: "The problem is growing worldwide due to the dropping of trade barriers."
She also stressed how easy it was to produce fake medicines. In some instances flour has been put into capsules and passed off as medicine.
The WHO believes that the manufacture of fake medicines is largely a cottage industry, with most production taking place in people's backyard rather than in large warehouses.
It said the problem had mushroomed in part due to a lack of deterrent legislation in many countries.
International agencies including the WHO and Interpol began a three-day meeting in Hanoi on Tuesday to try to tackle the multi-million dollar problem in southeast Asia.
Fake medicines are a growing concern in the Mekong countries of
Cambodia, China, Laos, Myanmar, Thailand and Vietnam, where they
undermine health programmes, according to the WHO.