People who use unqualified dental operators are "gambling with their health", the British Dental Association (BDA) has warned.
It follows the conviction of Samuel Irvine Madine at Belfast Magistrates' Court on Wednesday.
Madine admitted the use and supply of illegal tooth-whitening products.
The BDA said using the products he was selling would be like "brushing your teeth in bleach".
Madine was previously convicted in April 2015 of illegally impersonating a dental practitioner and carrying out teeth whitening.
His most recent conviction came after a case was brought by Belfast City Council.
Madine pleaded guilty to five charges relating to the use of and supply of tooth-whitening products.
He also admitted failing to meet the obligations of a distributor, including the required safety and labelling requirements.
Officers from the council's environmental health department seized four different products from Madine over a period of time for testing.
The products were found to contain the banned substance sodium perborate, which is carcinogenic, and also contained excessive levels of hydrogen peroxide.
A spokesperson from the BDA said: "It is alarming that a back street operator has been able to flout the law since his first prosecution in 2015.
"This fraudster was convicted for levels of hydrogen peroxide higher than dentists are legally permitted to use.
"It's the equivalent of brushing your teeth in bleach.
"Such high levels of hydrogen peroxide are likely to cause irreparable harm to teeth and gums, including blisters and burns to the gum and damage to nerves and tooth enamel.
"This man's reckless disregard for his customer's health is confirmed by his use of sodium perborate, a carcinogenic substance which is banned across Europe."
The case has been adjourned until 4 June for sentencing.