Abstract Background Recruitment for randomised controlled trials in palliative care can be challenging; disease progression and terminal illness underpin high rates of attrition. Research into participant decision-making in medicinal cannabis randomised controlled trials (RCTs) is very limited. Nesting qualitative sub-studies within RCTs can identify further challenges to participation, informing revisions to study designs and recruitment practices. This paper reports on findings from a qualitative sub-study supporting RCTs of medicinal cannabis for symptom burden relief in patients with advanced cancer in one Australian city. Methods Semi-structured qualitative interviews were conducted with 48 patients with advanced cancer, eligible to participate in a medicinal cannabis RCT (n=28 who consented to participate in an RCT; n=20 who declined). An iterative and abductive approach to thematic analysis and data collection fostered exploration of barriers and enablers to participation. Results Key enablers included participants’ enthusiasm and expectations of medicinal cannabis as beneficial (to themselves and future patients) for symptom management, especially after exhausting currently approved options, and a safer alternative to opioids. Some believed medicinal cannabis to have anti-cancer effects. Barriers to participation were the logistical challenges of participating (especially due to driving restrictions and fatigue), reluctance to interfere with an existing care plan, cost, and concerns about receiving the placebo and the uncertainty of the benefit. Some declined due to concerns about side-effects or a desire to continue accessing cannabis independent of the study. Conclusions The findings support revisions to subsequent medicinal cannabis RCT study designs, namely, omitting a requirement that participants attend weekly hospital appointments. These findings highlight the value of embedding qualitative sub-studies into RCTs. While some challenges to RCT recruitment are universal, others are context (population, intervention, location) specific. A barrier to participation found in research conducted elsewhere—stigma—was not identified in the current study. Thus, findings have important implications for those undertaking RCTs in the rapidly developing context of medical cannabis.