Celecoxib was one of the drugs found to be available online
UK experts have warned of the dangers of purchasing drugs online after a study showed wide availability of strong painkillers over the internet.
A team at Edinburgh University found 35 websites selling prescription-only pain drugs to UK customers without requiring a prescription.
Government regulators warned that buying medicines online could have "deadly" consequences.
The results appear in the British Journal of Clinical Pharmacology.
By trawling for online pharmacy sites through Google and Yahoo search engines, the researchers found almost 100 selling medicines to UK consumers.
Almost half of those sold prescription-only painkillers but 76% did not require the customer to provide a valid prescription.
Six asked for a prescription but accepted a fax or email copy which the researchers said could be forged or modified.
The drugs available included opioids such as tramadol and non-steroidal anti-inflammatories, such as naproxen and celecoxib, commonly prescribed for conditions such as arthritis.
They can have severe side-effects and some should not be taken in conjunction with other medicines or in people with certain conditions, the researchers said.
At best you will be wasting your money and at worst they could be deadly
Co-proxamol - a drug which has been removed from the market in the UK due to concerns over the risk of overdose - was available from three websites.
The researchers did not take the final step of actually buying the medicines as they felt it would not be ethical.
Study author Professor David Webb, an expert in clinical pharmacology at the University of Edinburgh, said these were drugs that people should not be taking without advice from a doctor, but they were available with a couple of clicks of the mouse.
"We were surprised at how many sites you could go to and find, in some cases, quite potent painkillers that have addictive potential and that you could get them without prescription."
He added: "The regulators are very aware of this problem and it is not one they can easily manage.
"But there certainly needs to be some sort of international agreement."
A spokeswoman for the Medicines and Healthcare Products Regulatory Agency, said although the sale of prescription only medicines without a proper patient consultation was illegal it was not illegal to buy medicines online.
"But we strongly advise people that buying drugs or slimming products from the internet is not a good idea, unless buying from a website that has a Royal Pharmaceutical Society of Great Britain (RPSGB) internet pharmacy logo.
"At best you will be wasting your money and at worst they could be deadly.
"You don't know what these products contain and you don't know in what conditions they have been made."
The RPSGB launched their logo scheme for bone fide pharmacy websites at the beginning of 2008.
Priya Sejpal, RPSGB head of professional ethics, said: "Internet pharmacy undoubtedly offers patients easy access and choice, but it also brings increased risk.
"There are a growing number of unregulated suppliers operating online with no professional qualifications or healthcare expertise.
"In many cases, medicines purchased from such suppliers are counterfeit, substandard or unapproved new drugs."
Dr Joan Hester, consultant in pain medicine at King's College Hospital in London and president of the British Pain Society said she had not heard of patients buying painkillers on the internet but they were clearly widely available online.
She warned that long-term use of opioids could lead to addiction and serious side-effects.
"In the right dose at the right time opioid analgesics can undoubtedly improve function and quality of life, but in large constant doses, especially if unmonitored, the effect of them can be disastrous."
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