Abstract: There is presented a wearable device (200) and corresponding system and method for sensing a substance ingested by a subject, for instance an intoxicating substance. The wearable device has a sensing arrangement extending between a first port (205a) and a second port (205b). The sensing arrangement includes two chambers, a pump and two flow-control devices. The first chamber is adapted to collect gas from a skin region of the subject. The second chamber has a sensor to sense the substance. The first flow-control device is provided at the first port (205a) and the second flow-control device is provided at the second port (205b). A controller is provided to control the pump and the sensor. The controller is operable in a first phase to sense a first gas sample from ambient environment, and a second phase to sense a second gas sample from a skin region of the subject.