Instrumental activities of daily living (IADL) are significant health indicators closely related to executive functions and able to detect mild cognitive impairment. A decline in IADL usually precedes ADL limitation, including taking medications, and may therefore predict a cognitive decline. We aimed to investigate the association of patients' IADL score with other clinical factors, with a particular focus on the presence of a caregiver, and the impact on adherence to androgen receptor pathway inhibitors (ARPIs) and survival outcomes within the Meet-URO 5-ADHERE study. It was a large prospective multicentre observational cohort study monitoring adherence to ARPIs in 234 metastatic castrate-resistant PC (mCRPC) patients aged ≥ 70. We observed an association between impaired IADL and lower geriatric G8 scores (p < 0.01), and lower adherence to ARPIs whether assessed by pill counting (p = 0.01) or self-reported by the patient himself (p = 0.03). The combination of an IADL < 6 and the absence of a caregiver resulted in a significantly high risk of non-adherence to the ARPIs at the multivariable analysis (HR 9.23, 95% confidence interval 2.28-37.43, p = 0.01). IADL alongside the geriatric G8 scales represent essential tools to identify frail and less auto-sufficient patients who are extremely vulnerable particularly if not supported by a caregiver and have the highest risk of nonadherence to ARPIs.