Background: Two randomised controlled trials (RCTs) conducted simultaneously in the same Irish university teaching hospital have shown that provision of Screening Tool of Older Persons’ Prescriptions (STOPP)/Screening Tool to Alert doctors to Right Treatment (START) recommendations to attending prescribers by a physician or a pharmacist can reduce in-hospital adverse drug reactions (ADRs) in older adults (≥ 65 years). The aims of this study were to compare the prescriber implementation rates of STOPP/START recommendations between the physician approach and the pharmacist approach in these two RCTs and to provide a narrative summary of the comparable clinical outcomes. Methods: Data were extracted from the two RCT published papers and their associated computerised databases to calculate the percentage prescriber implementation rates for the STOPP/START recommendations. The Chi-square test was used to quantify the differences in prescriber implementation rates, with differences considered statistically significant where p < 0.05. Results: Prescriber implementation rates of the STOPP and START recommendations made by the physician were 81.2% and 87.4% respectively, significantly higher than those made by the pharmacist (39.2% and 29.5% respectively), p < 0.0001. A greater absolute risk reduction in patients with ADRs was shown with the physician’s intervention compared to the pharmacist’s intervention (9.3% vs 6.8%). Conclusion: This study shows that the methods of communication and the medium through which the STOPP/START recommendations are delivered significantly affect their implementation. Non-implementation of some pharmacist-delivered recommendations may be contributing to preventable ADRs in older adults. Thus, future research should aim to identify the factors influencing prescriber implementation of pharmacist recommendations in order to inform the design of more effective pharmacist interventions in optimising older patients’ pharmacotherapy.