Long infamous for its extremely strict drug laws, Thailand did a drastic U-turn in 2018 when it became the first country in Southeast Asia to legalise medical cannabis (กัญชา ganchaa), expanding this to legalise recreational use as well. While extracts of cannabis including all edibles are limited to under 0.2% THC, there are no limits on the potency of plant parts like flowers, leaves or stems. Consumption in public is not permitted, but is now legal in private homes and any spaces like cafes and bars that choose to allow it. Vaping, importing and exporting all remain illegal.
Kratom (ต้นกระท่อม ton krathom), a local plant whose leaves are chewed for their mildly stimulant effects, was also fully legalized in 2021.
Thailand continues to maintain an extremely hard line on all other drugs, particularly methamphetamine (ยาบ้า ya ba, "crazy medicine"), which is a major social problem. Your foreign passport is not enough to get you out of legal trouble. Possession and trafficking offenses that would merit traffic-ticket misdemeanors in other countries can result in life imprisonment or even death in Thailand. Police frequently raid nightclubs, particularly in Bangkok, with urine tests and full body searches on all patrons. Ko Pha Ngan's notoriously drug-fueled Full Moon Parties also often draw police attention.
Penalties for drug possession in Thailand vary in harshness depending on the following: category of drug, amount of drug, and intent of the possessor. If you do take the risk and get arrested on drug-related charges, you would do well to immediately contact your embassy as a first step. The embassy cannot get you out of jail but can inform your family back home of your arrest, and can often give you a list of lawyers and translators you can contact.
The Thai legal system limits the amount of the defending and the usefulness of a lawyer. For minor offences, the penalty can be something like a 2,000-baht fine and deportation. However, to actually get this sentencing, someone not familiar with the system would need external help, translations, bail posted, etc. Stay clean, so you won't have to worry about penalties.