Background Intensive care unit (ICU) patients receive numerous interventions, but knowledge about potential interactions between these interventions is limited. Co-enrolment in randomized clinical trials represents a unique opportunity to investigate any such interactions. We aim to assess interactions in four randomized clinical trials with overlap in inclusion periods and patient populations. Methods This protocol and statistical analysis plan describes a secondary explorative analysis of interactions in four international ICU trials on pantoprazole, oxygenations targets, haloperidol and intravenous fluids, respectively. The primary outcome will be 90-day all-cause mortality. The secondary outcome will be days alive and out of hospital in 90 days after randomization. All patients included in the intention-to-treat populations of the four trials will be included. Four co-primary analyses will be conducted, one with each of the included trials as reference using a logistic regression model adjusted for the reference trial's stratification variables and for the co-interventions with interactions terms. The primary analytical measure of interest will be the analyses' tests of interaction. A p-value below .05 will be considered statically significant. The stratification variable- and co-intervention-adjusted effect estimates will be reported with 95% confidence intervals without adjustments for multiplicity. Conclusion This exploratory analysis will investigate the presence of any interactions between pantoprazole, oxygenation targets, haloperidol and amount of intravenous fluids in four international ICU trials using co-enrolment. Assessment of possible interactions represents valuable information to guide the design, statistical powering and conduct of future trials.