The changes follow Prime Minister Prayuth Chan-ocha's call for a crackdown on narcotics after a former policeman, who was discharged from duty for drug use, went on a knife and gun rampage last October, killing 37 people including 24 children.

The Thai health ministry on Thursday issued a regulation to classify those found in possession of even one methamphetamine or "yaba" pill as drug dealers rather than users, meaning possession of small amounts could be subject to severe penalties including imprisonment.

Currently, drug dealers face a prison term of up to 15 years or between two and 20 years if found selling to minors.

But those caught with smaller amounts could avoid prison in favour of rehabilitation or treatment.

Last year the health ministry proposed only people in possession of more than 15 pills should be classified as dealers.

"The changes to the ministerial regulation is to address social problems in a definitive and effective way and to curb the spread of yaba pills," Health Minister Anutin Charnvirakul said.

He added law enforcement officials could continue to exercise discretion on a case-by-case basis.

The latest regulation is pending cabinet approval.

Experts say the new approach risks reversing reforms introduced in 2021 when parliament passed a bill prioritising prevention and treatment over punishment for small-time drug users.

The government said that legislation had resulted in reducing prison sentences for almost 50,000 prison inmates.

"If a one-tablet limit goes ahead, already overcrowded prisons will endlessly fill, there won't be any space to hold the people classified as dealers," said Jeremy Douglas, Southeast Asia and Pacific representative of the United Nations Office on Drugs and Crime.

"The country is flooded with meth - this is not the time to reverse drug reforms."

Australian Associated Press