The number of illegal drug users rose by 130,000 this year after Thailand legalised cannabis for medical purposes, the Office of the Narcotics Control Board (ONCB) said.
ONCB secretary-general Niyom Termsrisuk blamed easy access to the drug for the rising number of narcotics users.
The previous coup-installed government passed a law last year to legalise marijuana for medical purposes only. However, cannabis remains a Narcotic Type 5, with steep penalties for its misuse.
Despite cannabis being available to prescribed patients only, the policy has led to cannabis becoming a readily available source of recreational drugs, he said, adding that it was easy for teenagers to obtain it.
He said the underground market was using the positive image of cannabis to boost their online trading efforts.