Purpose <p>OM is a very relevant and sometimes therapy-limiting side effect of CT/RCT. There are prophylactic and therapeutic measures available that should be recommended to all patients. This study investigated how patients were informed about oral mucositis (OM) as possible side effect of CT/RCT, how well they knew about available prophylactic and therapeutic measures from the clinical guidelines and to what extent they applied these.</p> Methods <p>A standardized questionnaire on information and usage of prophylactic and therapeutic measures and patient-relevant outcomes based on the German S3 Guideline was distributed among patients in German cancer departments.</p> Results <p>Only 61.6% of 114 patients were informed about OM as possible side effect by their physician and 53.2% had complaints caused by OM. An insufficient number of patients were recommended to apply prophylactic and therapeutic measures. 63.5% of the patients felt well-informed about treatment options. The most frequently recommended measure was mouth rinse (50%). Only 17.6% of patients were advised to visit a dentist.</p> Conclusions <p>The measures proposed in the German S3 guideline were insufficiently recommended. To improve patient education and the quality of care, more intensive use should be made of information flyers, training of nursing staff and greater interdisciplinary cooperation. If treatment-associated OM is to be expected, dental consultations should be firmly integrated into treatment planning.</p>