Abstract: On-board ILS sensors for detecting illegal drugs and based on intracavity laser spectroscopy (ILS) are provided for detecting the presence of drugs and their metabolized by-product vapors in an enclosed space, such as a vehicle. The sensor comprises: (a) a laser comprising a gain medium having two opposed facets within a laser resonator and functioning as an intracavity spectroscopic device having a first end and a second end, the first end operatively associated with a partially reflecting (i.e., partially transmitting) surface; (b) a reflective or dispersive optical element (e.g.