Medication safety in hospitals is a major patient-safety priority, particularly where high-alert drugs require timely laboratory interpretation and rapid dose adjustment. Pharmacists and nurses jointly influence these processes through ordering, sampling, administration, monitoring, and escalation. This systematic review evaluated how laboratory-guided services supported by pharmacist–nurse collaboration affect medication safety outcomes in hospital settings. We focused on hospital-based interventions involving anticoagulation management, therapeutic drug monitoring (TDM) of antimicrobials, anti-Xa-guided thromboprophylaxis, and renal dose adjustment programs. We searched major databases for open-access original studies evaluating pharmacist-led or pharmacist–nurse collaborative models with explicit laboratory-linked endpoints. Outcomes included appropriateness of monitoring, dose adjustment accuracy, surrogate safety outcomes, and error reduction. Nine eligible original studies were included. In settings, pharmacist managed protocols were associated with improved adherence to evidence-informed dosing pathways, more appropriate laboratory monitoring, and increased attainment of therapeutic targets. The effect was most consistent in inpatient warfarin protocols and vancomycin TDM programs. Renal dose adjustment policies supported by clinical pharmacy review reduced clinically meaningful dosing errors. Anti-Xa–guided approaches supported by pharmacist input improved the likelihood of reaching target prophylactic ranges in high-risk trauma populations.