The present disclosure is related to a method and apparatus for determining THC usage of a person. The present disclosure describes acquiring a video sequence, of an eye of a patient, the video sequence being a plurality of video frames, determining a frequency spectrum from a pupillary data of the video sequence, and determining, based on the frequency spectrum, the physiological characteristic or drug of use of the patient. In an embodiment, at least one frequency can be probed based on which physiological characteristic is being explored. For example, the physiological characteristic can be Δ⁹-tetrahydrocannabinol, and the at least one frequency probed can be selected to be specific to Δ⁹-tetrahydrocannabinol.
