Marijuana is illegal in Thailand and even carrying small amount of it can bring a person to jail. The penalty of illegal possession of cannabis, receiving and buying it is three months to five years imprisonment and a fine. Heavier penalties may be imposed if ever you will be caught cultivating or trafficking pot.
Reference: marijuanatravels.com (2016). Thailand, Country, Laws and legal news about marijuana in Thailand. Available from: < http://www.marijuanatravels.com/countries/tha >. [2 Feb 2016].