From a simple possession to distribution to drug trafficking, I have handled hundreds of these cases. They might arise from a car stop, a house raid, a wire tap, or an observation by a police officer.
There are usually a wide array of issues to litigate; was it a proper stop, was it a proper search, was the search warrant defective, was there adequate probable cause. Other issues need to be explored such as the chain of custody, the analysis, and on and on. Any one of these issues can be the key to a dismissal or not guilty verdict.