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Rationale Radial artery spasm (RAS) is a common complication during transradial cerebral angiography (TRA), but currently, the optimal prevention strategy is not well established. Papaverine has anti-vasospasm, sedative and analgesic effects. However, the efficacy of papaverine in preventing RAS during TRA remains unknown.Aims To assess the efficacy of papaverine in preventing RAS during TRA.Sample size estimates 240 participants will provide 80% power at a two-tailed significance level of 0.05 to test the superiority hypothesis with the assumption that the incidence of RAS is 4% and 14.62% in the intervention and control groups, respectively, allowing for a 5% drop-out rate.Methods and design The Efficacy of Papaverine to Prevent Radial Artery Spasm During Transradial Cerebral Angiography is a multicentre, randomised, placebo-controlled, double-blind trial. Eligible participants scheduled for TRA are 1:1 randomised to the intervention group and the control group. The intervention group will be slowly injected with 10 mL of papaverine hydrochloride solution through the successfully implanted radial artery sheath and continuously dripped papaverine hydrochloride solution through the artery sheath during the procedure, and the control group will be given normal saline in the same manner.Study outcome The primary outcome is the incidence of RAS during the procedure. The operator assessed RAS based on a questionnaire addressing the following five signs: persistent forearm pain, pain response on catheter manipulation, pain response to introducer withdrawal and difficult catheter manipulation after being ‘trapped’ by the radial artery with considerable resistance on withdrawal of the introducer. RAS was indicated by the presence of at least two of these five signs or by the presence of just one when the operator considered it necessary to administer other spasmolytic agents.Trial registration number NCT05861765.