The Behaviour Change Wheel is a novel way to use behavioral theory to make interventions. In this document, we explain how we created an intervention to help general practitioners (GPs) better manage medications for patients with multiple conditions. We did this by looking at the results of previous research, doing our own original research, and using the Behaviour Change Wheel, all while following the UK Medical Research Council's guidelines for complex interventions. Adhering to the Behaviour Change Wheel framework, we investigated behaviors related to medication management in multimorbidity using a systematic review and a qualitative research including general practitioners. We chose one of the changeable GP behaviors we found and did a specific behavioral investigation to figure out why GPs were or were not doing this behaviour. We used behavioral analysis to identify the intervention functions, behavioral change approaches, and implementation strategy most likely to induce behavioral change. In the systematic review and qualitative analysis, we found several GP behaviors that might be changed. From these, we picked active medication review (instead of just keeping things the same) as the behavior we wanted to change. Behavioral study elucidated the skills, possibilities, and motivations of general practitioners about active medication review. We put together the three intervention functions that were most likely to modify behavior (enablement, environmental restructuring, and incentivization) to produce the MultimorbiditY Collaborative Medication Review and Decision Making (MY COMRADE) intervention. The main part of MY COMRADE is social support, which is when two GPs look over the drugs supplied to a patient with many health problems together. Four more behavioral modification approaches are included: changing the social environment, prompts/cues, action planning, and self-incentives. This study is the first to use the Behaviour Change Wheel to create an intervention for multimorbidity. It also shows that the approach can be used and is useful in a complicated area of clinical care. The methodical creation of the MY COMRADE intervention will enable a comprehensive assessment of its efficacy in the subsequent phase of this project.