(This is an edited version of an answer I gave on quora.com.)
Thinking of travelling to Thailand? Here are a few things you should know about the law.
Drugs are available (or so I’m told) but illegal. Punishment for possession, use and trafficking of drugs ranging from marijuana and the local favourite, “yaba”, up to hard drugs can be much more severe than in the West, including very long spells in prison and even the death penalty (rarely carried out, but still on the books).
Sex: Prostitution is widespread but in a legal grey area, so be careful. You are unlikely to be punished for a transaction with a sex worker, but (quite rightly) pedophilia and sex trafficking carry harsh penalties. Bear in mind that “lady boys” are very common in the sex trade. Know what you are getting yourself in for.
Defamation/ lese majeste. You can be fined and sent to jail for up to 14 years for insulting the King or other immediate members of the Royal family. Even making what you may think is a joke or satirical remark of the kind that is common, say, in Britain, could lead to jail. It is even an offence to read certain books, visit some websites, or follow certain people on social media.
People are obliged to report any instance of lese majeste they encounter, and many Thais will do so. Be very careful of what you say, even in what you may think is friendly company.
“Normal” defamation against another person is also a tricky area since an imputation is defamatory if it is deemed hurtful to the person’s feelings or their reputation. It does not matter if it is true, it can still be defamatory.
Disrespecting religion. Don’t get a Buddha tattoo; don’t pose nude or in a disrespectful way with a religious icon or at a place of worship.
Road rules: The traffic is crazy; accidents are common; the fatality rate is high. Make sure you have a licence from your own country, plus an international licence (available from motoring organisations), and that you are licensed for the vehicle you are riding. If you don’t have a motorcycle at home, you can’t ride one in Thailand.
Wear a helmet if riding and make sure you have comprehensive insurance — enough to get you the best hospital treatment in Thailand, and for medivac if it’s necessary.
If you are in an accident, the system may seem a little biased against you. You may be asked for a bribe at some stage. Use your discretion and common sense. Don’t be overly invested in your own innocence if the matter is trivial. It’s best to pay a small fine and move on than to get caught up in the legal system, which will only delay your travel and could lead to a world of pain.
Otherwise, just behave yourself. Be tolerant and respectful to everyone. Don’t get involved in fights, don’t get mindlessly drunk, take normal safety precautions. Have fun.