In my home county in the UK, recently, they caught more people drug-driving than drink-driving. (The road side test can test for cannabis or cocaine). The penalty's the same, a minimum 12 month driving ban.
The one difference being, in the UK, as in most Western countries, it's best not to offer a bribe to the cops to try to get off with it. (It's well known that Thailand has a weak 'rule of law').