The Centre of Addiction Studies has found the number of people aged under 20 who use cannabis recreationally has doubled since the plant was formally decriminalised this year.

The increase has caused concern among critics of the policy over the plant's impact on the physical and psychological health of young people, especially as the effort to pass the cannabis and hemp control bill is currently going nowhere.

CADS director said the increase in recreational cannabis use among Thai youth underlines the need to closely monitor the impact of the policy.

A lecturer with Prince of Songkla University's Faculty of Medicine said while the negative impacts of cannabis use are still manageable, there has been an increase in the number of patients seeking treatment after consuming cannabis.

Patients seeking treatment for psychological side-effects following cannabis consumption account for about 3% of all ER admissions nationwide last year, but this year, the figure is closer to 17%, the lecturer said.

The Office of the Narcotics Control Board said there are growing concerns that locally-grown cannabis which does not meet medicinal standards might go to the black market to meet demand by recreational users.

Up to 95% of domestically cultivated cannabis fails to meet the standards required for further processing.