A system and method for drug screening for testing an iris for use of a chemical known to have a neurological effect that interferes with normal functioning of the iris and the voluntary muscles of the eye. The system and the method involve testing and documenting reactivity of the iris under known light conditions and can test for the presence of horizontal nystagmus. The identity of a subject can be confirmed by applying an iris pattern recognition model of a captured image of an iris of the eye compared with an image of an iris stored in a library of baseline color or a library of infrared eye images. The system and the method involves providing alarms when one eye pupil diameter exceeds or falls below the opposite eye pupil diameter and when horizontal gaze nystagmus has been determined to be present.