I've been to Thailand many many times for long times at a time I also speak the language to some extant so I've got some insight in the drug scene there. Methamphetamine is known there as Yaba, which mean crazy drug, it is almos always smoked some do eat it and it has meth and caffeine, when it first came around it called horse drug, because you know meth makes you very good worker at the first at least, the government forced the name change to yaba, so not to make people think it makes you strong like horse. At the beginning maybe 20 years ago yama actually contained heroin with meth so you can imagine how addictive it was.
There is said to be 4 million methaddicts in the country, many of them are taxi drivers, bus drivers and other workers. Also some years ago also crystal meth came to market, its more expensive but became very popular. For local people yaba might cost around 200 baht (so its about 5$) sometimes even lower, for foreigners (farangs) it might cost 1000 baht so 25$, crystal for local people costs around 400-600 baht so around 10-15 dollars but for farangs around 50-100 dollars. Of course if you have local friends who use it they will get it in local prices. The snitch culture is very strong there, if someone gets caught with one yaba pill the police will encounter him to do more purchase from that dealer and after that they come arrest the dealer, thais will always do the fake buying I think almost never they refuse because you get smaller punishment if you do that. If you have over 4 yaba pills or 0,5g crystal meth you might get punished as dealer and can get you jailed for 10 years, for smaller possession its mostly one day in jail and fine. They do random piss tests in night club and any party street and you will get taken to police station and get fine as long you don't have any drug with you.
There are lots of fake dealers in night club and street who are doing work with police, they might sell you real drug and after 5 min police will come to arrest you, if you want to bribe you're way out you have to pay at least 40 000 baht, so around 1000 dollar. The only safe way to buy and consume drugs in Thailand is having some local friend who uses it also, he will get the drug and you go to smoke it with him in his home, you should stay there as long the effects will go down. Never take any drugs with you or to youre hotel.
Heroin is available, usually nigerians are the dealers at least for foreigners, but with local contact he can get you it prices are around 60-100 dollar, I almost bought but was too paranoid about it so I passed. The purity is usually very high, because usually it comes from BUrma where they product it and it is very pure and clean. In Brma there is huge problems with heroin, even police and army person might do it, they gather with other users and shoot it up. Opium can also be founded among hilltribes in the Chiang Mai or Chiang Raai, Thailand break down poppy cultivation in 50's because most hill people men were smoking it heavily., but even nowadays there is still people smoking it in the northern Thailand.
Tramadol is/was very popular amongst thai teens, they usually take it with promathizine cough medicine, but because many teens went to convulsion the new junta began to crack down its sell, now its very hard to get any tramadol from pharmacy. About 4 years ago you could buy 50mg x 10 or 100mg depot tramadol from airport pharmacy, EVERY pharmacy sold it every western owned pharmacy sold it. In smaller pharmacies 50mgx10 IR tramadols could cost as little as 50c (20baht), they almost never limited the amounts, I once bought 10 packets of 100mg depot tramadol and 10 packets of 50mgx10 IR tramadol. If you go now there its very hard to get, some pharmacies ask you're passport and need to write you're information to documents which they send to government. Some pharmacies might sell you most benzos, ketamine, buprorphine and methylphenidate but the prices are very high. Also ketamine and exstacy are very popular among locals. Also lyrica and gabapentin are sold in any pharmacy also in airport. Also you migh get codeine 30mg/500 paracetemol content, some pharmacies will sell you pure 30mg codeine tablets.
I know that cannabis use is popular among youth, but I'm not in to that drug so I don't know, one time I smoked joint with very nice and friendly thai guy, he even offered me meal and said I can sleep in his place if I cant find questhouse.
You should not do any drugs there if you dont speak the language and dont know local people, there are lots of farang methaddicts who sell it and get caught and sent to prison for 10 years.
Also in southern Thailand its very easy to get kratom, many older workers use it, lately the new junta is thinking about making use of kratom more legal in medicine use. The new junta is planning to end the harsh drug laws and even make possession of small amounts of meth for personal use decriminalized or at least not to sent to jail but to rehab.
Also in Thailand there is very successful Buddhist temple for drug addicts, it involves lots puking and meditation, it was established in 70's when there was huge opium and heroin use boom, but nowadays most people are methaddicts. They say around 80% dont go back using drugs. Its very good place. Thais in general are very nice people.
One thing I noticed, there are lots of long term meth users but Ive never seen any in so bad shape like some american methheads are. Even one older man who used meth for 20 years daily, he still had his teeths and face had some acne but nothing else.
Also IV meth use is very rare, I asked many people about it and they were shocked they told me they don't do that here. In the other hand there are heroin addicts who shoot it up. In Bangkok and some other cities there are methadone clinics for heroin addicts, I don't think there is any limit for patients, but they get their week doses for once, so no need to go there every day.