Background Improper disposal of domestic unused and expired medicines is a source for environmental contamination. This pharmaceutical pollution pathway is also an under-recognized cause of antimicrobial resistance (AMR). There is a need for standardized assessment tools for quantifying disposal behaviour at the population level. Methods A methodological study was conducted to validate the MeDisPract, the Medication Disposal Practices (MeDisPract) tool. Following literature review and expert consultation, a 14-item instrument was finalized with three primary domains: behavioural (practice), cognitive (knowledge), and attitudinal (readiness). Following content validity, data were collected from 509 households over 25 villages. Reliability was evaluated using ordinal alpha, split-half reliability, Guttman Lambda-6 and Cronbach’s alpha (for Likert rated items). Construct validity was assessed using exploratory factor analysis. A weighted composite MeDisPract Index (0–100) was constructed to categorize disposal risk. Results The tool demonstrated good reliability for a behavioural surveillance instrument (Ordinal α≈0.78, Guttman Lambda-6≈0.75); Cronbach’s α for four Likert items of the behavioural domain (0.57) is acceptable for a multidimensional public health instrument. Factor analysis supported a five-domain structure (KMO=0.604 which is above cut-off indicating acceptable sampling adequacy; Bartlett p<0.001), explaining 68% of total variance. Index scores showed expected heterogeneity across households, with a substantial proportion of households reflecting unsafe disposal behaviours despite good attitude towards safer disposal of medications. Conclusions MeDisPract provides a pragmatic, field-deployable instrument to quantify medication disposal behaviour and identify communities at risk of pharmaceutical environmental exposure. The index can support AMR action plans, environmental health surveillance, and evaluation of medication take-back interventions in low and middle income settings.