Pharmacist-led transitions-of-care interventions can improve medication processes, yet effects on downstream utilization and sustainability vary across settings and patient groups. Conventional intervention labels do not adequately explain why apparently similar services produce different outcomes. To develop and refine a realist programme theory explaining how patient complexity, interprofessional integration, information continuity, patient capability, and organizational capacity condition mechanism activation in pharmacist-led transitions of care. A realist review of peer-reviewed evidence published during 2017–2026 used iterative, programme-theory-driven evidence identification and selection for relevance and rigour. Evidence was examined as context, intervention resource or opportunity, mechanism, outcome, rival explanation, and boundary condition. Contradictory and null findings were retained to test and refine explanatory configurations. The synthesis indicates that pharmacist involvement is not a mechanism in itself. Benefits are more plausibly generated when pharmacist resources produce usable medication information, timely opportunities for collaborative action, patient understanding and confidence, and continuity across care boundaries. These mechanisms may be weakened by regimen and patient complexity, fragmented responsibility, poor information transfer, workflow constraints, or insufficient organizational support. Improvement in medication discrepancies may therefore coexist with little change in utilization, while some higher-need subgroups may derive greater benefit. A context-sensitive explanation accommodates variable transition outcomes better than a uniform intervention-effect assumption. The refined theory identifies clinically and organizationally important activation conditions while remaining an explanatory synthesis rather than a validated prediction rule. Transferability depends on setting, population, intervention fidelity, outcome definition, and the evidence supporting individual context–mechanism–outcome configurations.
