Background: The significance of the project is related to the vulnerability as a central condition for reflecting on practices and policies related to evidence-based health care. Objective: To analyze the influence of physical and social vulnerability before and after the implementation of evidence for evaluation, prevention and management of oral mucositis (OM) in patients with head and neck cancer (HNC), undergoing radiotherapy. Method: Adverse effects to radiotherapy were compared, before and after the implementation of evidence by the JBI method, in vulnerable adult and elderly patients with HNC, receiving radiotherapy, associated or not with chemotherapy. The comparison between the phases was performed by statistical tests, according to the distribution of the variables, using SAS 9.4 and considering a significance level of 5%. The study was approved by the Research Ethics Committee of Unicamp under the number CAAE: 47636621.0.0000.5404. Results: 48 patients were included in the study, 23 (47.9%) before and 25 (52.1%) after implementation; in both periods there was a predominance of men (80.1%-84.2%; p 1.000), elderly (median 63a-68a; p 0.073), with low schooling (52.4%-52.6%), smokers (2.9%-78%) alcohol consumption (71.4%-63.2%; p 0.577), with more than one comorbidity, in addition to the HNC. There was no significant percentage reduction in sociodemographic characterization and health history between periods. The reasons related to complications in radiotherapy treatment were associated with more than one factor for the same individual; before: OM (90.48%), radiodermatitis (66.67%), odynophagia (23.81%), dysphagia (23.81%), asthenia (14.29%) and hematological conditions (9.52%); after: radiodermatitis (73.68%), OM (47.37%), odynophagia (26.32%), dysphagia (26.32%), asthenia (10.53%) and hematological conditions (5.88%). There was a significant percentage reduction (p 0.003) of OM between the periods before and after the implementation of evidence. Conclusions: Despite social and physical vulnerability, OM presented a significant percentage reduction after the implementation of evidence for evaluation, prevention and management of OM in patients with HNC. Implications for practice: Vulnerable people can benefit from best practices for reducing OM, one of the adverse effects of radiation therapy treatment that can provide malnutrition, hospitalization, interruption in cancer treatment and compromise the quality of life of the patient.