Police obviously don't have free rein to arrest people and seize evidence to prosecute them. If police question suspects or seize evidence in a manner contrary to law or the Constitution, it could make the case fall apart. What defenses are available to Johnson County defendants charged with drug offenses or other crimes?
First, an obvious defense is that the defendant is not guilty of the crime. Either no crime was committed, or someone else committed it. Prosecutors must prove beyond a reasonable doubt that the defendant committed the crime. If the prosecution fails to convince a judge or jury that the defendant is guilty beyond a reasonable doubt, the defendant must be found not guilty.
A second defense is that no evidence of the crime can be introduced in court because the authorities acquired it improperly. For example, police must inform persons in custody of their Miranda rights before questioning them. If the police don't do this, the suspect's statement - and any other evidence arising from the statement - are inadmissible in court. If police wish to avoid committing an illegal search and seizure, they must procure evidence either with a warrant or with a legitimate exception to the warrant requirement. For example, if contraband is in plain view of the police, it may be covered by a warrant exception.
There are many other situations that could constitute a defense to an alleged crime. One fairly rare occurrence is entrapment. This happens when police induce a defendant to commit a crime they otherwise would not have committed. Although it is uncommon for this defense to apply, the defense does exist and does apply occasionally.