The two men, aged 20 and 21, were targeted by the same dealers who were involved in the death of another Briton last month, confirmed police in Netherlands, reported BBC.
Police discovered the bodies in a hotel room on Tuesday (25 November) with traces of a white powder.
Police spokesperson Rob van der Veen said: "We did a quick test to find out what kind of powder it was. It wasn't cocaine, it was an opiate. In this case, white heroin.
"We took the sample to the Dutch forensic institute and they found out the contents were the same as material that we seized in other cases where people got sick."
A post-mortem examination later this week will confirm exactly how the two men died.
So far, police officials have not released the victims' names.
A Foreign Office spokesman confirmed that they're liaising with the Dutch authorities and "providing consular assistance to the family."
Der Veen said an estimated 17 people have reportedly fallen victim to white heroin recently. "White heroin looks like cocaine, is sold as cocaine and people think they are snorting cocaine. The result is respiratory failure."
Authorities have warned tourists against purchasing anything from drug dealers on the streets, and been distributing leaflets since October across hotels, hostels and the Amsterdam's central railway station.
One leaflet reads: "[In] the last couple of weeks several tourists [have been] hospitalised with severe health problems after using cocaine bought on the street. Research has revealed that the cocaine used was not cocaine, but white heroin."
The leaflets also remind people that in Amsterdam "you won't be arrested for taking drugs."