By Tulip Mazumdar
Newsbeat health reporter
Police show off fake Viagra sold in 11 countries and seized in a raid in China
One in 10 men may be "literally gambling on life" by buying potentially fake medicines, mostly off the internet.
The latest warning comes from Pfizer Inc, one of the world's biggest pharmaceutical companies and the maker of Viagra, who say 90% of drugs sold online are thought to be counterfeit.
The drug is used to help men who have problems getting an erection.
It's one of a number of medicines men are getting off the internet because they don't want to discuss it with anyone.
Others include things like weight loss drugs and anti-depressants.
They are all medicines that should be given out with a prescription so that a pharmacist or doctor can talk people through how it works and the possible side effects.
Dr David Gillen from the company said: "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."
While it may come as no surprise that a major pharmaceutical company is warning people off drugs not made by them, the advice is being backed by the Medicines and Healthcare Products Regulatory Agency.
Naeem Ahmed is the head of intelligence at the MHRA.
He said: "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.
"You do not know what these products contain and you do not know in what conditions they have been made."
There are a number of legitimate and perfectly safe online pharmacies which should have a Royal Pharmaceutical Society of Great Britain logo on the front page.
But the advice from the MHRA is speak to your doctor first.