The directive, signed by Public Health Minister Cholnan Srikaew on December 8, was published in the Royal Gazette on Friday with effect on Saturday.
According to the new law, sellers of kratom leaves or food spiked with the herb are required to display a clear sign showing that the products will not be sold to anyone under 18, pregnant women and breastfeeding mothers.
The directive also said the size of the sign must correspond with the shop’s size.
Online sellers are also required to display warnings clearly on their sales pages or websites.
Kratom leaves, or Mitragyna speciosa, have been traditionally used for pain relief and energy boost, and are popular among labourers.