Bangkok's nightlife is infamously wild, but it's not quite what it used to be: due to recent social order campaigns, there have been quite a lot of crack-downs on opening hours, nudity, drug use etc. Nearly all restaurants, bars and clubs are now forced to close at 1AM sharp, although a few are allowed to stay open till 2AM. Informal sidewalk bars do stay open all night, particularly in Sukhumvit and Khao San Road. You must carry your passport for ID checks and police occasionally raid bars and discos, subjecting all customers to drug tests, though these mostly occur at places that cater for hi-society Thais.
One of Bangkok's main party districts is Silom, home not only to perhaps the world's most famous go-go bar strip Patpong, but plenty of more legitimate establishments catering to all tastes. For a drink with a view, the open-air rooftop bars of Vertigo and Sirocco are particularly impressive. A large number of superhip and more expensive bars and nightclubs can be found in the higher sois of Sukhumvit, including Bed Supperclub, Q Bar, and Narcissus, as well as the hip area of Thong Lo (Soi 55).
Hippie hangout Khao San Road is also slowly gentrifying and a score of young trendy Thai teenagers have also made their mark there. Going out in Khao San Road is mostly casual, sitting at a sidewalk bar watching people pass by, but some nightclubs have opened here as well. Most of the younger Thais prefer to congregate around Ratchadaphisek.
Smoking is forbidden in all restaurants, bars and nightclubs, whether air-conditioned or non-air-conditioned. Remarkably for Thailand, this rule is strictly enforced.