Men were found to like the anonymity of buying medicine on the web
Men are being warned they could be "gambling with their lives" if they buy fake medicines online.
Dr David Gillen, medical director for the drug company Pfizer, warned men were more likely to "bypass" the NHS in order to obtain treatments anonymously.
A survey by Pfizer of 900 men over 35 found one in 10 had bought prescription medicines without authorisation.
The UK's medical regulator said buying medicines online was not illegal, but added it was "not a good idea".
The report says medicines for "lifestyle" conditions such as erectile dysfunction, such as Viagra, and weight loss drugs have been traditionally targeted by those who create fake medicines.
But it warns UK authorities have recently seized counterfeit versions of heart attack and cancer treatments.
Of the men who had bought prescription-only medicines without a prescription, half used the web to do so.
A third cited convenience and speed as the key reasons for buying medicine without prescription.
But the report warns up to 90% of all medicines sold on the internet are thought to be fake.
It said: "It seems that men are turning a blind-eye to the health risks associated with purchasing counterfeit medicines from unregulated sources and are particularly naive about the ingredients that counterfeit medicines might contain."
It adds that fake versions of drugs can be almost impossible to spot, but can containing inactive ingredients like talcum powder, or even toxic ingredients such as rat poison.
'Convenience and anonymity'
Dr Gillen said: "Men bypassing the health system to purchase medicines is a growing problem in the UK, particularly relating to the increased availability of counterfeit medicines.
"These new findings show that men are not only often ignorant about what medicine actually requires a prescription, but worryingly they know buying medicine from illicit sources might be harmful - but convenience and anonymity often outweigh their fear levels."
Fake versions of erectile dysfunction drugs, like Viagra, are sold online
Naeem Ahmed, head of intelligence at the MHRA, said: "People are advised that they should only take prescription-only medicines after an appropriate consultation with their GP.
"Only suitably qualified healthcare professionals can take into account risks and benefits associated with medicines.
"Anyone who self-medicates and buys their medicines from internet sites could be in danger of receiving counterfeit or substandard medicines.
"At best these will be a waste of money, at worst they can harm or even kill."
He said anyone who suspects they have been offered or sold counterfeit medicines should contact the MHRA, and added: "It is not illegal to buy medicines online but we strongly advise people that buying medicines 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."