More than 2m people regularly buy medicine over the internet
People should not buy anti-flu drug Tamiflu over the internet without a prescription, experts have warned.
The government's chief medical officer said Britain had a "massive" stockpile of genuine Tamiflu.
The drug, used in suspected cases of swine flu, has now overtaken Viagra as the subject of the most junk e-mails, the Royal Pharmaceutical Society says.
But the supplies offered are often fake, with buyers getting sugar or rat poison instead of the drug.
Sir Liam Donaldson said in an interview with BBC Breakfast: "The whole field of counterfeit drugs is becoming a much bigger problem, not just with Tamiflu.
"So my advice is don't buy it , you don't need to. We have got the biggest stockpile in the world and even worse than that you might end up with something that is poisonous and dangerous."
He said the NHS was well prepared to treat the increasing number of swine flu cases that would come with the winter.
"As we move towards the flu season we will see many more cases. That's what we expected. People should not be concerned."
He said: "The NHS will be stretched, it will be under pressure. But we have good plans in place both locally and nationally to deal with this."
Sir Liam confirmed a swine flu vaccine would be available towards the end of August and he said there would be enough for half of the population through the winter, up to Christmas.
The Royal Pharmaceutical Society's research suggests more than two million people regularly buy medicine over the internet. However, much of that trade is legal.
A spokesman, David Pruce, said: "We now think that Tamiflu is the most spammed medicine on the internet. It's taken over from Viagra. Most of that Tamiflu could well be fake. If it's fake it could range from simple sugar to rat poison."
Meanwhile, the government says the rising numbers of swine flu cases mean trying to contain the virus is no longer an option.
Ministers said the emergency response would now move to a new "treatment" phase across the UK as there may soon be 100,000 new cases a day.
Consequently, anti-flu drugs will no longer be given to the close contacts of those infected nor will lab testing be done to confirm cases.
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