Family caregivers face many problems during administration of medications. Pharmacists are reliable and accessible healthcare providers in addressing family caregivers' medication related inquiries in primary care. This study aimed to evaluate theshort-term outcome of apersonalized theory-based pharmaceutical care service on the medication administration problems of family caregivers. This pre-post intervention study was carried out among family caregivers at a community pharmacy in Istanbul, Türkiye from May through December 2022. The service was developed by mapping problems using theTheoretical Domains Framework and selecting related behavioural change techniques. The primary outcome was the change in the scores on the Turkish version of the Family Caregiver Medication Administration Hassles Scale (FCMAHS-TR) from baseline to the one-month follow-up assessment. Secondary outcomes were their ability to read health related materials, care burden, and satisfaction. Among family caregivers (n = 100), total score of FCMAHS-TR was significantly reduced after receiving theory based pharmaceutical care service (median [IQR]23.0 [17.0-27.0] vs 17.0 [10.3-20.0]; p < 0.001). The proportion of family caregivers with high ability to read health related materials was significantly increased (34.0% vs 48.0%; p < 0.05) with non-significant change in the proportion of family caregiver with high burden (97.0% vs 94.0%; p > 0.05). Younger family caregivers had significantly higher scores on the Turkish version of Patient-Oriented Pharmacy Services Questionnaire (PSPSQ) 2.0 when compared with the older ones (≥ 65 y) (p < 0.05). Family caregivers' medication administration problems werereduced after receiving thetheory based pharmaceutical care service. Long term impact and national implementation of this service should be assessed in the further studies.