Flexible wearable chemical sensors are emerging tools which target diagnosis and monitoring of medical conditions. One of the potential applications of wearable chemical sensors is therapeutic drug monitoring for drugs that have a narrow therapeutic range such as lithium. We have investigated the possibility of developing a fibre-based device for non-invasive lithium drug monitoring in interstitial fluid. A flexible cotton-based lithium sensor was coupled with a carbon fibre-based reference electrode to obtain a potentiometric device. In vitro reverse iontophoresis experiments were performed to extract Li+ from under porcine skin by applying a current density of 0.4 mA cm-2 via two electrodes. Carbon fibre-based reverse iontophoresis electrodes were fabricated and used instead of a conventional silver wire-based version and comparable results were obtained. The fibre-based Li+ sensor and reference electrodes were capable of determining the Li+ concentration in samples collected via reverse iontophoresis and the results compared well to those obtained by ion chromatography. Additionally, biocompatibility of the materials used have been tested. Promising results were obtained which confirm the possibility of monitoring lithium in interstitial fluid using a wearable sensor.