"Under section 12 of the Drugs Act, B.E. 2510 (1967), no person shall manufacture, sell, import or order into the kingdom any modern drug unless such person receives a license from the licensing authority. Thus, bringing drugs or ordering drugs to be shipped into Thailand by post is not allowed."
and
"In the case of a traveler traveling into Thailand, you may bring with you any modern drug for personal use for no longer than 30 days according to section 13 (4) of the Drugs Act, B.E. 2510 (1967) and its amendments. The drug should be kept in its original container and should be accompanied by its prescription from the doctor."
FDA THAI : Food and Drug Administration, Thailand (moph.go.th)