Background: Oral mucositis is a common inflammatory complication among patients undergoing hematopoietic stem cell transplantation (HSCT). Among its therapeutic properties, Chamomilla recutita has anti-inflammatory effects. Objective: The aim of this study was to identify the dosage of the liquid extract of C recutita in mouthwash that is needed to reduce the incidence and intensity of oral mucositis in adult patients undergoing allogenic HSCT. Methods: In a randomized phase II clinical trial, 40 patients were randomized to receive routine care plus mouthwash containing a liquid extract of C recutita at 0.5%, 1%, or 2% (experimental groups) or standard care alone (control group). Daily evaluation was performed using the measurement scale for oral toxicity defined by the World Health Organization. Statistical analysis was performed, in which the incidence, intensity, and duration of oral mucositis were compared between each experimental group and the control group. Results: The experimental group at the 1% dosage demonstrated reduced incidence, intensity, and duration of oral mucositis compared with the control group. The formulation was well tolerated by patients and was safe, as no moderate or severe adverse effects were identified. Conclusions: In this study, the use of mouthwash containing 1% C recutita extract can be associated with reduced incidence, intensity, and duration of mucositis in adults patients undergoing allogenic HSCT. Implications for Practice: The results of this investigation will help nurses and other professionals in selecting the C recutita dosage used to manage oral mucositis in patients undergoing HSCT.