There are a wide range of possible means by which you can defend against drug charges. Your lawyer will look at the unique factors in your case to decide how to defend against the accusations levied against you. Some defenses could focus on a failure to observe your constitutional rights because of an unlawful search or lack of probable cause.
There could also be possible procedural issues, like a chain of custody problem or a misidentification of the substance. There are possible defenses around whether the prosecution can successfully prove the drugs were in your possession.