Since 2013, once a medicine receives marketing authorisation in the European Union, it is labelled with an inverted black triangle indicating all adverse reactions should be reported. Our aim was to explore understanding of the black triangle and compliance with adverse event (AE) reporting requirements by UK oncology healthcare professionals (HCPs). A questionnaire was electronically distributed to oncology pharmacists (P) via the British Oncology Pharmacy Association, to oncologists (O) through the Association of Cancer Physicians and also to nurses (N) via the UK Oncology Nursing Society. Overall, 125 (42 O, 61 P, 22 N) clinicians participated. The purpose of the black triangle was unknown by 26% (55% O, 5% P, 28% N) and 54% did not alter their AE reporting in the presence of a black triangle. Once the black triangle was removed, only 38% were aware which AEs should be reported, 46% did not report all serious AEs for established medicines, including life-threatening or disabling AEs. Reasons for non-reporting were decision making on what to report (45%); time consumed by reporting (41%); AEs perceived as not serious enough (35%) and follow-up process (23%). Understanding of the pharmacovigilance framework among respondent groups was variable. Across all groups, AEs appear substantially under-reported. Reasons identified in the study include the time consuming nature of AE reporting and a lack of understanding around the black triangle and AE reporting process. There is a need to further support HCP education on AE reporting coupled with a review of the current reporting process to ensure maximal engagement.