AMSTERDAM — Health authorities in Amsterdam are warning of dangerous drugs being sold to tourists after three British men died in the past month after snorting heroin they may have believed was cocaine.

A large sign outside the Dutch capital’s Central Station warned Thursday, “Extremely dangerous cocaine is sold to tourists” in the area. Similar signs were set up at other popular tourist haunts in the historic city.

The warning campaign was started after two British men, aged 20 and 21, were found dead in a hotel room in the city on Tuesday. Police say they likely died after snorting “white heroin” they probably bought believing it was cocaine.

Amsterdam police spokesman Rob van der Veen said the same drug is suspected in another British tourist’s death last month.

A further 14 people have become sick after taking the drug, Van der Veen added.

He said it is not yet clear why heroin is apparently being sold as cocaine. A team of police is investigating the case and trying to track down the dealer involved.