Leading drug company Pfizer is warning consumers over the dangers of taking counterfeit prescription drugs.
Counterfeit Viagra is sold over the internet
The company which makes Viagra warns that what people buy might not match the description on the packet.
John Theriault, head of global security, will tell a London conference held by think-tank Civitas, of the dangers of buying over the internet.
He will also warn about "parallel packaging", where drugs made in one country are repackaged for another.
This is done if medicines are imported for sale in the UK from other countries.
Around 140 million medicines are parallel traded each year in the European Union, with some 70% destined for the UK.
It is illegal to sell medicines where the labelling and instructions are in a foreign language, so importers have to add English labelling.
Companies such as Pfizer are warning that if this is done by putting the drugs into completely new packaging, there is the danger of drugs being tampered with or counterfeit versions reaching the consumer.
However, no such cases have come to light in the UK.
Pfizer has now introduced new packaging security measures for medicines sold in Europe.
'Go to the GP'
The company has also warned about the dangers of buying medicines over the internet.
A spokesman for Pfizer said: "In one case which came to light, one man in China was dealing in counterfeit Viagra, supplying over a quarter of a million pills a month.
"He had infiltrated about nine European and Middle Eastern countries and around 10 US states."
People in the UK had bought the fake Viagra, he said.
"There is an enormous amount of counterfeit drug available," he added. "Because it is a prescription drug, people do buy it, but they could end up with anything."
He said people must go to their GP in order to obtain the genuine drug.
'No definite evidence'
A spokeswoman for the Medicines Healthcare products Regulatory Authority said cases referred to it involving counterfeit products were relatively rare, with no definite evidence of any cases since one involving the ulcer drug Zantac in 1994.
She added: "The MHRA has had no definitive evidence that counterfeit pharmaceuticals are reaching the general public via the legitimate supply chain, eg through pharmacies.
"However, there is some evidence that with counterfeit medicines could reach the public through mail order or through internet purchases."
She said the MHRA investigated any reports it received of websites which appeared to be in breach of regulations regarding advertising, or sale and supply of medicines.
In 2003, counterfeit Viagra with an estimated value in excess of £2.35m was seized.
"The MHRA already has systems in place to receive and investigate reports of packaging and labelling problems with medicines, including parallel traded products."
She said the World Health Organisation estimated around 6% of medicines available globally were counterfeit.
Dr Chaand Nagpaul, a member of the British Medical Association's GP prescribing committee said: "I do see patients who have purchased drugs over the internet.
"It tends to happen in relation to drugs like Viagra, slimming drugs and sleeping tablets, where buying things privately might be attractive."
But he added: "I am concerned about the lack of safeguards for buying drugs that way."