I do not use weed, but here is from the government page shared by @Weathervane above. You must have a medical prescription to buy weed from June 2025 forward.
https://thailand.prd.go.th/en/content/category/detail/id/2078/iid/401785
In a significant shift, Thailand's Ministry of Public Health has
officially restricted cannabis use exclusively to medical purposes,
rolling out new regulations aimed at preventing misuse and ensuring
its benefits are channeled solely for health.
Under the revised framework, medical access requires an authorized
prescription, limited to a 30-day supply. Patients must obtain their
cannabis through a licensed medical professional for individual
treatment.
And it appears the government will further regulate it in 2026 according to this article.
According to a draft of an amended Health Ministry regulation on
“controlled herbs,” sales of cannabis will only be allowed at medical
facilities, stores selling herbal products, pharmacies, and through
traditional medicine practitioners.
Establishments must also have at least one practitioner, who has
completed training from the Department of Thai Traditional and
Alternative Medicine, on duty during operating hours, according to the
draft regulation.
Since mid-2025, buyers must obtain medical prescriptions to buy
cannabis and dispensaries are required to sell only cannabis produced
by farms that have obtained the “Good Agricultural and Collection
Practices” certifications issued by the government. Sales are capped
at a 30-day supply per customer.