Purpose <p>Population ageing has transformed intensive care unit (ICU) populations, with older adults increasingly presenting with multimorbidity, polypharmacy, and frailty. This review aims to examine how these interrelated constructs shape vulnerability to critical illness and influence clinical decision-making and outcomes in geriatric critical care.</p> Methods <p>We conducted a narrative review of the literature focussing on the interactions between multimorbidity, polypharmacy, and frailty in critically ill older adults, and their implications for medication-related harm and care delivery across the ICU trajectory.</p> Results <p>Multimorbidity drives clinical complexity and prescribing burden, contributing to high rates of polypharmacy. Polypharmacy is associated with adverse drug events, functional decline, and mortality. Frailty modifies risk through altered pharmacokinetics, heightened susceptibility to treatment-related harm, and the depletion of adaptive capacity that characterises the syndrome. These factors interact dynamically during critical illness, particularly at transitions of care, where medication-related harm is most likely to occur.</p> Conclusion <p>In older ICU patients, multimorbidity, polypharmacy, and frailty interconnect and interact to shape both risk and recovery. Recognising frailty supports more nuanced, patient-centred decision-making beyond survival alone. Integrating medication stewardship strategies, including reconciliation, deprescribing, and multidisciplinary review, offers a practical approach to reducing harm and improving outcomes that matter to older patients.</p>