Dutch politie (police) are pretty relaxed and helpful unless you do something clearly wrong, such as littering or smoking a joint right under their noses.
Police can hold offenders for up to six hours for questioning (plus another six hours if they can’t establish your identity, or 24 hours if they consider the matter serious). You won’t have the right to a phone call, but they’ll notify your consulate. You’re presumed innocent until proven guilty.
Anyone over 14 years of age is required by law to carry ID. Foreigners should carry a passport or a photocopy of the relevant data pages; a driver’s licence isn’t sufficient.
Technically, marijuana is illegal. However, possession of soft drugs (eg cannabis) up to 5g is tolerated. Larger amounts are subject to prosecution.
In April 2018 Den Haag became the first Dutch city to officially ban smoking cannabis in its city centre, train station and major shopping areas.
Don’t light up in an establishment other than a coffeeshop without checking that it’s OK to do so.
Hard drugs are treated as a serious crime.
Never buy drugs of any kind on the street; fatalities can and do occur.
Prostitution is legal in the Netherlands. The industry is protected by law and sex workers pay tax. Much of this open policy stems from a desire to undermine the role of pimps and the underworld in the sex industry.
In Amsterdam’s Red Light District you have little to fear as the streets are well-policed, but the back alleys are more dubious.