Men were warned yesterday about a fake version of a drug for impotence.
They were told to return the counterfeit packets to chemists who supplied them.
An investigation has been launched into how fake Cialis, a successful product developed by Eli Lilly, has entered the legitimate medicine supply system.
The Medicines and Healthcare products Regulatory Agency (MHRA) said yesterday that the counterfeit tablets did not "pose an immediate risk".
How many packets of the fake Cialis are in circulation is unknown as the source is illegal. The real product costs about £20 for four tablets.
The MHRA has identified two fake lot numbers, A031410 and A041410, which are printed at the end of the carton and on a blister strip inside. Both are for 20mg tablets. The yellow tablets and the packaging have been carefully copied.
The fake tablets contain the active drug tadalifil, but concern about their safety is based on lack of quality control.
Ely Lilly said the fraud had come to light when a customer tried to split a tablet in two and found, unusually, that it disintegrated.
A company spokesman said: "This is not Cialis. It would seem that counterfeiters have persuaded wholesalers or other middle men to buy from them."
