Background: Medications for symptom management in palliative care have associated, but poorly understood, harms. Drug-related harms have important clinical implications, may impact on patients' compliance and contribute to symptoms. Objective: To explore the longitudinal relationship between oral morphine equivalent daily dose (MEDD) and oral diazepam equivalent daily dose (DEDD) with functional, cognitive, and symptom outcomes in patients receiving palliative care. Design: Secondary longitudinal analysis of cancer decedents (n = 235) was carried out from a palliative care randomized controlled trial with multiple outcome measures. At each time point, MEDD and DEDD were calculated. Multilevel modeling was used to investigate independent associations between MEDD and DEDD, and cognitive and gastrointestinal symptoms, quality of life (QoL), performance status, and survival. Setting/Subjects: Participants were recruited from a specialist palliative care program in southern Adelaide, were expected to live ≥48 hours, had pain in the previous 3 months, and a baseline Folstein Mini-Mental Status Examination score ≥25. Results: Cognitive and gastrointestinal symptoms, performance status, and QoL worsened over time. In the adjusted multilevel analysis, statistically significant relationships remained between MEDD/DEDD and worsening performance status (p = 0.001), DEDD and gastrointestinal effects (p < 0.001), MEDD and QoL (p < 0.022). Conclusions: Commonly used palliative medications were associated with deteriorating performance status. The lack of association between MEDD with gastrointestinal or cognitive symptoms underlines that these associations are not inevitable with close attention. This analysis highlights the importance of including other medications as confounders when exploring medication-related harms. An understanding of the risk-benefit balance of medications is needed to maximize net benefits for patients.