Each case is ultimately different and the potential defenses have to be determined by an experienced criminal defense lawyer based on the facts of your case. However, some of the more common defenses to drug possession cases are:
1. Improper stops (seizures): Under the law, police need a "reasonable suspicion" to stop you. They must have some kind of reasonable belief that you have committed, are about to committ, or have committed a crime or offense. If the police stop you withou the required "reasonable suspicion" anything they may ultimately find may be subject to suppression by the court. If the evidence is "supressed" then there is no evidence against you and the case will likely be dropped by the state or thrown out by the court.
2. Improper searches: Even if the police lawfully stop you, the police still need "probable cause" to search you or your property (unless you give them consent). If a police officer searches or seizes your property without probable cause and finds contraband, then that contraband may also be subject to suppression by the court. If suppressed, the evidence can't be used against you and the charges may end up being dismissed or dropped.
3. Lack of Actual or Constructive possession: Actual possession means you have contraband on you. Actual possession is defined as:“Actual possession” of a controlled substance exists where a defendant has physical possession of contraband and knowledge of such possession. So, if you are not in actual possession of contraband, the state may try to prove that you had "constructive" possession of the contraband.
Many times a person will be arrested if they are in a car with several people and police find contraband in the car. The police, if nobody claims possession of the contraband may arrest everyone! If the police find contraband in a place (car, house, etc.) where more than one person has access, dominion, and control of the contraband, and you didn't know that the contraband was there, or you didn't know that the contraband was illegal, then you may have a defense that you did not have actual or contructive possession of the contraband and therefore, can't be charged with possession. To establish defendant’s constructive possession, the State is required to prove three essential elements: (1) his dominion and control over the contraband; (2) his knowledge that the contraband was within his presence; and (3) his knowledge of the illicit nature of the contraband.”
A lawyer after analyzing your case and taking the deposition of officers may be able to file a "motion to suppress" evidence the state claims they have against you. If successful, the evidnence can't be used against you and the charges may be dismissed or dropped.