BACKGROUNDSurgery for breast cancer is common, and intravenous opioids are often used to control postoperative pain. Recently, pectoralis-2 (PECS-2) block has emerged as a promising regional anaesthetic alternative. With nomenclature recently proposed, this block is termed combined PSP/IPP-block (pectoserratus plane block/interpectoral plane block).OBJECTIVEWe aimed to compare the need for postoperative rescue morphine between the intervention group that received a pre-operative combined PSP/IPP-block and a control group that received peri-operative long-acting opioids for postoperative analgesia.DESIGNA randomised controlled study.SETTINGOperating theatres of two Swedish hospitals. The patients were recruited between May 2017 and October 2020.PATIENTSAmong the 199 women scheduled to undergo breast cancer surgery (sector resection or radical mastectomy) who were enrolled in the study, 185 were available for follow up.INTERVENTIONAll patients received general anaesthesia. The intervention group received a combined PSP/IPP-block before surgery. The control group received intravenous morphine 30 min before emergence from anaesthesia.MAIN OUTCOME MEASUREThe primary endpoint was the cumulative need for intravenous rescue morphine to reach a predefined level of pain control (visual analogue scale score <40 mm) during the first 48 h after surgery.RESULTSData from 92 and 93 patients in the intervention and control groups, respectively, were analysed. The amount of rescue morphine administered in the 48 h after surgery was significantly lower in the intervention group than in the control group (median: 2.25 vs 3.0 mg, P = 0.021). The first measured pain score was lower in the intervention group than in the control group (35 vs. 40 mm, P = 0.035). There was no significant difference in the incidence of nausea between the groups (8.7 vs. 12.9%, P = 0.357).CONCLUSIONThe use of a combined PSP/IPP-block block before breast cancer surgery reduces the need for postoperative rescue morphine, even when compared with the use of intra-operative morphine.TRIAL REGISTRATIONClinicaltrials.gov identifier: NCT03117894.