Now that Illinois has approved medical marijuana cards for prescribed users, the question is "under what circumstances can a card holder be prosecuted for DUI?" The marijuana card places the requirement upon the person to submit to a blood draw/urine sample should they be requested to do so by law enforcement in a DUI scenario. And there is immunity granted to such a person for DUI prosecutions.
The requirement to submit to the chemical testing of the officer's choice is mandatory. Failure to comply results in imposition of an immediate Statutory Summary Suspension and revokes immunity from prosecution of the DUI. There is really no reason for a card holder targeted for DUI to refuse to submit. The only possible reason would be to hide other illicit drugs from being detected in one's system. If that is true, then immunity from DUI prosecution would be unavailable.