Police have confirmed the presence of carfentanil in Guelph.
A drug trafficking investigation lead to the arrest of two men back in June.
Officers say the heroin they seized was sent to Health Canada for analysis and it was determined that they contained fentanyl and carfentanil.
Carfentanil is similar to fentanyl but can be 100 times more toxic. Even a grain, the same size as salt, could be fatal to humans.
This is the first time the drug has been found in Guelph.