Background: Antiadhesion products are essential for postoperative care in patients after thyroidectomy by providing a physical barrier to cover the exposed tissue and thus preventing abnormal adhesion of adjacent tissues. Since thyroidectomy may result in swallowing difficulties arising from damage or inflammation of the surrounding tissues, the use of antiadhesion agents such as MegaShield® or Guardix-SG® will help reduce scar formation. This may thus improve postoperative swallowing function in patients. Methods: Patients were enrolled and followed up between October 4, 2018, and March 26, 2020. Patients during the postoperative follow-up sessions were randomly allocated to the standard care with Guardix-SG® and clinical trial medical device application group with MegaShield® (test group) in a 1:1 ratio by the permuted block randomization method. Patient performance on penetration aspiration scale (PAS), National Institutes of Health-Swallow Safety Scale (NIH-SSS), videofluoroscopic dysphagia scale (VDS), Dynamic Imaging Grade of Swallowing Toxicity (DIGEST) based on Videofluoroscopic swallowing study (VFSS) were collected. Nonadhesion-reducing agent patient data were used as a control group. Results: No statistical significance was shown ( P > .05) between the 2 groups of MegaShield® and Guardix-SG® in various phases from thick semisolid, thin semisolid to liquid for both PAS and NIH-SSS. Several statistical significances were reported in the results comparing various criteria of PAS, NIH-SSS, VDS at different oral and pharyngeal phases, and DIGEST in all 3 stages among MegaShield®, Guardix-SG®, and nonadhesion-reducing agent group. Conclusions: These results prove the noninferiority of MegaShield® compared with Guardix-SG® as an antiadhesion agent in postthyroidectomy care.