Lack of compliance with regard to medical prescriptions has become a major cause of treatment failure, a problem particularly faced in the aging population. In order to aid patients in taking their medication properly, various products for solid drugs like electronic pill organizers have recently appeared on the market whereas there is a lack of effective solutions for liquid drugs. In this paper, a novel direct printed RF sensor for drug bottles is presented as a low-cost solution, which can be used for monitoring liquid medication. The fully-printed, single layer metal sensor has been realized on the top of a polyethylene bottle, acting as a resonant LC tank, which can be read remotely. Resonant frequency drifts proportionally to the liquid content, since the distributed capacitance value is affected by the permittivity of the liquid. The demonstrator presented in this article shows an impressive sensitivity up to 316 kHz/mL, resulting in a costefficient and viable solution to detect content variation in the order of sub - milliliters.