Thailand’s prime minister, Srettha Thavisin, said on Wednesday that he wanted cannabis to be re-listed as a narcotic by the end of the year, a reversal two years after the country became one of the first in Asia to decriminalize marijuana.

Mr. Srettha, who took office in August, wrote on social media that he wanted the Ministry of Health to change the laws so that marijuana could only be used for medical and health reasons.