Medical regulators seized 3.5million unlicensed erection pills worth more than £10million at UK borders last year.
Photos show thousands of packets of fake drugs piled high - and the blue diamond shaped pills look almost identical to the real deal.
Viagra has been available over the counter or from a registers online pharmacy without a prescription since 2017 for around £4 a dose.
But men are still turning to websites which are not licensed because they sell pills which keep the penis erect for at a lower cost - as little as £1.
Copycat versions could be cut with toxic chemicals and have potential lifelong consequences, health chiefs warned.
Medical regulators seized 3.5million unlicensed erection pills worth more than £10million at UK borders last year.
Pictured, packets of fake pills with the blue diamond shape of Viagra
Photos show thousands of packets of fake drugs piled high
Viagra has been available over the counter without a prescription since 2017 for as little as £4.
But men are still turning to unregulated sources online because they are cheaper
Fake versions of impotence pills may look real, but fail to work in practice
Often illegal traders lure customers in with packaging almost identical to that on pharmacy shelves, posing as legitimate providers with better prices
The Government's Medicines and Healthcare products Regulatory Agency (MHRA) are warning men to steer clear of fake impotence pills.
Erectile problems in general affect up to one in five men in the UK, which is equivalent to 4.3million men.
Research suggests around 44 per cent of men with erectile dysfunction (ED) aged 40 and over have not sought medical help.
They may be buying replicas of drugs online that are readily available over the counter in pharmacies or given by prescription, the MHRA fear.