The opposition accused Public Health Minister Anutin Charnvirakul of causing social problems and violating local and international laws by decriminalising cannabis without proper controls.
Mr Anutin was the first to be grilled as the House began debating the opposition's motions of no-confidence in government ministers on Tuesday.
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Sutin Klungsang, the chief opposition whip from the Pheu Thai Party, began his attack with a video of Mr Anutin's election campaign promise that his Bhumjaithai Party would bring happiness to people by allowing them to grow cannabis, which would be decriminalised.
Cannabis would be sold and used to make food and prevent and treat illness, and even be smoked in private, Mr Anutin was seen saying in the video clip.
Mr Sutin said that speech violated Thai and international laws and also raised concern over the cannabis-smoking remark.
He said the cannabis policy violated the United Nations Office on Drugs and Crime's Single Convention on Narcotic Drugs of 1961, which Thailand ratified.
Cannabis was included as a dangerous narcotic in the convention, which limited its use to medical and research purposes and imposed strict controls on its use, he said.
Mr Sutin said the Public Health Ministry decriminalised cannabis on June 9 without laws to control its use and that was considered a violation of international law.
"The country could face international legal action over this issue," he said.
The chief opposition whip said the cannabis policy also violated Section 66 of the constitution, which required the country to cooperate with international organisations and protect the public interest.
Mr Sutin said that after cannabis decriminalisation on June 9, cannabis products and cannabis-infused products became available on the streets, students were seen abusing cannabis and people fell ill because of cannabis abuse and were taken to hospital.
Mr Sutin asked if the government could really protect young people from cannabis abuse while households were allowed to grow cannabis and Mr Anutin had already voiced support for cannabis smoking at home.
"Cannabis damages the brains of young people... There will be serious social problems," he said.
Mr Sutin also blamed Prime Minister Prayut Chan-o-cha for accepting Bumjaithai's cannabis decriminalisation policy as a condition of the party joining and supporting the coalition.
Medical, religious and judicial organisations had voiced their opposition to cannabis use, according to Mr Sutin.
