The pass, introduced in May, is a mandatory membership card for people wishing to purchase marijuana in coffee shops in the Netherlands.
It is meant to keep tourists from countries including Germany, Belgium and France from purchasing the drug at coffee shops, Radio Netherlands Worldwide reported Monday.
Researchers Nicole Maalste and Rutger Jan Hebben, in a study commissioned by Epicurus, a private foundation, tracked the effect of the weed pass and said they found the emergence of a large network of illegal street-sellers of marijuana and other drugs.
"We want cities and towns in the rest of the Netherlands to know what they can expect when the weed pass rules take effect there," said Maalste, a senior researcher at the University of Tilburg, referring to the government's intention of using the weed pass throughout the country.
The study found that illegal dealers are selling more than just marijuana, "raising the risk that young people buying cannabis will come into contact with hard drugs."