In Wisconsin, no person may possess or attempt to possess a controlled substance or analog unless the person obtains the substance or analog directly from or pursuant to a valid prescritpion or order of a practitioner who is acting in the course of his or her professional practice unless otherwise authorized to possess the substance.
Some common defenses to possession of a controlled substance include, no possession-it belonged to someone else. Imagine a house full of people and the police raid the house. All four are on the lease. It would not be hard to imagine the true owner of the drugs blaming it on another resident to avoid being charged and avoid going to prison. This is why it is important to hire a good criminal attorney when charged with a serious crime.
Other drug defenses may include suppression issues-the police did not have a warrant or exceeded the scope of the search or the warrant was stale, or the police lied to get the warrant (see Delaware v. Franks), evidence was improperly obtained, and evidence was lost.