Abstract Objective To compare the Liverpool Causality Assessment Tool versus Naranjo Scale for screening suspected adverse drug reaction (ADR) cases. Methods We retrospectively reviewed patient charts with a history of suspected ADR, scored using both instruments and determined how each correlates with laboratory and other investigations. 924 charts from the Clinical Pharmacology Clinic at the London Health Sciences Centre were reviewed and 529 charts contained objective findings to support or against the diagnosis of ADR. The participant age ranged from 1 month old to 93 years. We determined the sensitivity and specificity of Liverpool and Naranjo tools for predicting ADRs with scores ranging from “Possible” to “Definite” were considered positive and “Unlikely/Doubtful” as negative for ADR. These results were confirmed by laboratory or clinical (re-challenge) testing in 529 cases. Results Liverpool causality tool had sensitivity (SN) of 97.2% ± 2.4% and specificity (SP) of 2.3% ± 1.57%. The positive (PPV) and negative predictive values (NPV) were 34.1% and 61.5%, respectively. The Naranjo scale had SN of 81.2% ± 5.69% and SP of 13.2% ± 3.56%. PPV and NPV were 32.7% and 57.5%, respectively. Conclusions The Liverpool Causality Assessment Tool is a more sensitive tool than the Naranjo Scale in the assessment of possible ADRs but both tools have poor specificity. The Liverpool Tool can be a useful screening tool in settings where other tests may not be readily available. However the low PPV and NPV of both instruments suggests pursue further testing is needed to confirm or deny an ADR.