Provided according to embodiments of the present invention are methods of monitoring and treating respiratory depression that include securing a photoplethysmography (PPG) sensor to a central source site of an individual administering a central nervous system (CNS) depressant to the individual processing PPG signals front the PPG sensor with a computer in communication with the PPG sensor and administering a narcotic reversal agent to the individual if the PPG signals or a physiological parameter derived therefrom are outside a preset value range. Related systems are also described.
