People who are using marijuana for medicinal purposes can now register for legal possession without being punished.
Health ministerial regulations legalising possession of marijuana for medical, research and other purposes were published in the Royal Gazette on Wednesday.
They cover possession by patients who need marijuana to treat an illness, and the criteria for legal possession by governmental organisations, professionals, private universities, farm community-based enterprises, international transport operators and patients travelling internationally.
The 90-day registration period for legal possession began with the publication of the regulations on Wednesday. It ends on May 19.
Food and Drug Administration secretary-general Tares Krassanairawiwong said patients with any illness could register for marijuana possession if they had the relevant medical certificates.
Dr Tares said phone calls by people seeking more information had overwhelmed the FDA's 1556 hotline number.
Registration can be done at the FDA office in Bangkok and at provincial health offices.
The government has said repeatedly that recreational use of marijuana remains illegal.