In this case, the defendant passed out behind the wheel, leading to an accident. She had THC metabolites in her system, leading to an Aggravated DUI prosecution.
The defense wished to call the defendant’s personal physician to testify about the defendant low blood pressure. The doctor opined that that’s how it might have happened, but the doctor couldn’t say for sure.
The trial court barred this evidence. The Illinois Supreme Court affirmed, stating that if the defense evidence doesn’t establish that the unforseen medical condition was the sole proximate cause of the accident, then the evidence is improper.
The case is People v. Ida Way.
http://www.illinoiscourts.gov/Opinions/SupremeCourt/2017/120023.pdf