Aussie blokes looking for a bit of medicinal help in the bedroom have been advised not to turn to Rock Hard for Men because the product is illegal and poses a serious health risk.

They are warned not to use the erection aid because it contains "undeclared prescription substances".

The medicine regulator on Wednesday said although the United States manufacturer claims Rock Hard is 100 per cent herbal, tests show it actually contains the prescription-only substances tadalafil and glibenclamide.

It says commercial supply of the tablet product is illegal in Australia.