Medication-related risk after hospital discharge persists despite widespread use of medication reconciliation, pharmacist review, discharge counselling, electronic information transfer, and post-discharge follow-up. Their effects vary substantially across programmes and health systems, suggesting that intervention components alone do not explain whether transitional medication-safety work succeeds. This realist review examined how hospital-to-home programmes generate benefit, attenuation, or failure under differing conditions of workforce capability, information continuity, responsibility transfer, follow-up access, and patient or caregiver capacity. From 142 records or documents identified, 34 duplicates were removed, 108 unique records were screened, 54 underwent detailed assessment, and 34 substantive evidence documents were retained; 26 directly informed programme-theory development or refinement, while a separate methodological source informed realist appraisal. The synthesis found a recurrent process–outcome disconnect: medication discrepancies and prescribing problems could improve without corresponding reductions in adverse drug events, readmission, or other downstream outcomes. Explanations were strongest when medication information was timely and actionable, downstream professionals had the authority and capacity to respond, responsibility for unresolved medication work was explicit, follow-up was reachable, and patients or caregivers could enact the discharge plan. Weakness at any of these interfaces could interrupt otherwise technically successful interventions. Hospital-to-home medication safety is therefore best understood as a conditional health-system accomplishment rather than the automatic consequence of delivering a predefined transitional-care component.
