Drug charges can be dropped for a variety of reasons, some of which have been established from Supreme Court decisions. Many are the result of procedural errors by the police department.
If you are arrested for one offense, such as a weapons charge, and subsequent to a valid search are found to have drugs as well, the prosecutor will often drop one of the charges in exchange for a guilty plea and a negotiated sentence on the other. This saves the state time and the state money. In 2002, 95 percent of all felony convictions were a result of a plea agreement, according to Department of Justice statistics.
Drug charges are thrown out because of searches conducted without probable cause or reasonable suspicion, such as during a car search. If the police search your car subsequent to a routine traffic stop, and you gave them no other reason to believe you were potentially committing another offense, the search may be deemed illegal and the evidence obtained thrown out.
Cases of police officers convicted of shaking down citizens, or even drug dealers, puts prosecutors in the position of having to review all of the cases in which these officers were involved, as their credibility as witnesses is called into question. Prosecutors have, in the past, dropped pending charges and retried prior convictions in such cases.
If you are arrested for multiple offenses, and at least one is a felony, the prosecutor may drop the lesser charges and only prosecute the major charges.
If you are willing to give information to the police about others you were arrested with, or others involved in the drug trade, they may drop or reduce the charges against you in exchange for your testimony.