PURPOSETo evaluate the effectiveness of a calcium phosphate-containing-desensitizer (Teethmate Desensitizer - TD), caries type, subject age, and preoperative hypersensitivity on postoperative sensitivity (POS) after composite restorations on deep or extremely deep lesions.METHODS50 subjects, having two teeth with deep or extremely deep caries, participated in this study. TD was applied randomly to one tooth of each participant, and all teeth were restored with composite resin (Filtek Z250). After 1 week, POS was evaluated according to NRS (numerical rating scale) and VAS (visual analogue scale) by using participant diaries. At 6 weeks, POS was assessed considering subjects' reports. The normality of data was analyzed with Shapiro-Wilk test. For analyses, Pearson's chi-squared test, Mann-Whitney U and the Wilcoxon Signed-Rank test were used, and the effect sizes (ES) were calculated (α= 0.05).RESULTS47 of the participants completed the 6-week study. There was a small effect size noted for TD for NRS and VAS (P> 0.05, ES < 0.30). Also, there was no statistically significant difference between POS and subject age (P= 0.294, ES= 0.161), type of caries (P= 0.680, ES= 0.042) and preoperative sensitivity (P= 1.000, ES= 0.138) after the first week.CLINICAL SIGNIFICANCETeethmate Desensitizer had no significant effect on postoperative sensitivity occurrence with respect to caries type, subject age, and existence of preoperative sensitivity. The application of Teethmate Desensitizer demonstrated no significant relieving effect on postoperative sensitivity in deep or extremely deep cavities.