Intensity-modulated radiotherapy (IMRT) is a treatment method that is used in the treatment of head and neck cancers. Impairment of chewing and swallowing functions in the early and late periods of radiotherapy is frequent. Therefore, revealing the dose-effect relationship is important. The main purpose of this study is to investigate the dose-effect relationship between chewing and swallowing structures objectively via a standardized videofluoroscopy protocol. The study included 35 participants treated with chemo-IMRT. A videofluoroscopic swallowing study (VFSS) was performed before IMRT, and 3 and 6 months after IMRT. VFSS results were scored according to the Modified Barium Swallow Impairment Profile (MBSImP) and the Penetration-Aspiration Scale (PAS). Maximum interincisor mouth opening, body mass index (BMI), and Functional Oral Intake Scale levels were determined in these cases. The quality of life of participants was evaluated. There was a significant increase in PAS and MBSImP scores and a significant decrease in BMI scores of the patients after treatment. Xerotomy and sticky saliva complaints increased after treatment. The dose to the mastication muscles (> 40 Gy) and the temporomandibular joint (> 46 Gy) were found to be associated with a decrease in BMI; the dose to the superior pharyngeal constructor muscle (> 58 Gy) was found to be associated with pharyngeal stripping wave. The presence of aspiration was associated with the inferior pharyngeal constructor muscle, glottic larynx, supraglottic larynx, and upper esophageal sphincter. Important findings to emerge from this study include detected toxic dose limits. These findings may guide physicians to minimize the side effects of IMRT.