Purpose The purpose of this study was to explore the knowledge, attitudes and practices of health professionals working in clinical trials, to pharmacovigilance and adverse drug reaction (ADR) reporting. Methods A mixed methods study comprising an online questionnaire disseminated from September to November 2018, three semi-structured interviews and four focus groups. The qualitative components were conducted with a random sample of questionnaire participants who had provided their contact details (n = 24). The qualitative interviews were conducted at a location convenient to the participant's place of work between October and December 2018. Results One hundred forty-eight participants completed the questionnaire. Study coordinators/project managers represented the largest group of participants ( 28.6%, n = 38). Poor knowledge or understanding of ADR reporting was the most frequently cited barrier to ADR reporting (75%, n = 93). The most common enabler to reporting was having a clear understanding of an ADR definition (85.7%, n = 108). Focus group and interview participants described having limited staff as a barrier to reporting an ADR. They welcomed the prospect of pharmacovigilance training and indicated that face-to-face training would be preferred to provision of online training. Conclusion This study highlights key factors that influence the reporting of ADRs in clinical trials. Although the findings are specifically related to the clinical trial environment in Ireland, they may provide a useful platform for optimising the future conduct of trials. This research suggests that ADR reporting may be improved through provision of enhanced pharmacovigilance training to clinical trial staff.