Wis. Stat. § 939.47
Pressure of natural physical forces which causes the actor reasonably to believe that his or her act is the only means of preventing imminent public disaster, or imminent death or great bodily harm to the actor or another and which causes him or her so to act, is a defense to a prosecution for any crime based on that act, except that if the prosecution is for first-degree intentional homicide, the degree of the crime is reduced to 2nd-degree intentional homicide.
Wis. Stat. § 939.47
The defense of necessity was unavailable to a demonstrator who sought to stop a shipment of nuclear fuel on the grounds of safety. State v. Olsen, 99 Wis. 2d 572, 299 N.W.2d 632 (Ct. App. 1980). Heroin addiction is not a "natural physical force" as used in this section. An addict, caught injecting heroin in jail, who was not provided methadone as had been promised, was not entitled to assert necessity against a charge of possession of heroin because his addiction ultimately resulted from his conscious decision to start using illegal drugs. State v. Anthuber, 201 Wis. 2d 512, 549 N.W.2d 477 (Ct. App. 1996), 95-1365.