Cestur officers are generally friendly men and women whose job is specifically to help tourists. Many speak a little bit of a language other than Spanish. They wear white shirts with blue insignia and can usually be found near major tourist sights and centers. You should contact Cestur first in the event of theft, assault or if you are the victim of a scam, but you can equally ask them for directions to sights, which bus to take etc.
If a police officer stops you, be polite and cooperate; heavily armed roadway checkpoints aren't uncommon, particularly in regions bordering Haiti — they're looking for drugs and weapons.
You might also be asked for a 'tip' in cash or merchandise; feign misunderstanding or simply politely decline. More often than not, you'll simply be waved through. Buying drugs in the DR should be avoided. The seller is often in cahoots with the police who 'catch' the exchange in order to extract a bribe from unwary foreigners. Any transactions involving large amounts can result in significant prison time. Also worth noting is the reputedly 'impure' quality of the cocaine distributed in the DR — public service announcements warn that the majority is more dangerous chemical filler than anything else.
Female sex workers, when propositioning foreigners, are known to grab and touch aggressively, often a sly attempt at pick-pocketing. The legal drinking age in the DR is
