Background <p>Despite advancements in perioperative pain treatment for elective orthopaedic surgery, significant postoperative pain remains. Postoperative pain is a factor that influences recovery and hospital stay after prosthetic orthopaedic surgery. Treatment modalities, such as nerve blocks, local infiltration analgesia, and opioids, have been attempted. Each treatment modality has its own distinct advantages and disadvantages. Intrathecal morphine was administered in accordance with PROSPECT recommendations.</p> Objectives <p>Given the discrepancies between our findings and the recommendations, we retrospectively analysed the effectiveness and side effects of intrathecal morphine in patients undergoing hip or knee replacement.</p> Design <p>A single-centre retrospective analysis.</p> Setting <p>Large non-university teaching hospital.</p> Participants <p>A total of 854 patients who underwent spinal anaesthesia for knee or hip replacement surgery between March 15, 2022, and August 31, 2023, with or without intrathecal morphine. Data, registered as part of standard clinical practice, were collected from 854 patients, of whom 414 had received intrathecal morphine.</p> Main outcome measures <p>We collected pain scores on the day of the operation (day 0) and on the two days thereafter (days 1 and 2). Furthermore, side effects, including urinary retention requiring catheterization, nausea, vomiting, and itching, were studied.</p> Results <p>The pain scores were modestly, but significantly, lower on the day of the operation in patients receiving intrathecal morphine. However, on day two, the pain scores were modestly and significantly higher, in patients receiving intrathecal morphine. Additionally, patients in the spinal morphine group experienced more nausea, vomiting, and itching. Furthermore, they had an increased risk of catheterization in the univariate analysis (<i>P</i> &lt; 0.05). After correction, there was an increased risk of catheterization in males only (odds ratio, 1.63; 95% confidence interval: 1.01–2.63; <i>P</i> = 0.04).</p> Conclusions <p>The side effects of intrathecal morphine, with only a modest decrease in pain scores on the operation day, prompted us to stop administering intrathecal morphine for elective knee and hip replacement.</p> Clinical trial number and registry <p>This trial was registered in the Open Science Framework registry (<a href="https://osf.io/rt57a">https://osf.io/rt57a</a>).</p>