If you are a foreigner who is even suspected of looking for drugs, you will most certainly be set up by the undercover police. You will then be in for a very miserable, frightening and expensive experience.
These undercover police are everywhere. Unless you get a glimpse of a pistol, you have no idea which of the Dominicans you see on the street, on the bus, in the bars, etc., are undercover police looking for people who are ripe for the picking.
(Of course, in the DR the fact a person has a pistol doesn't necessarily mean he is a policeman.)