Background: The incorporation of chlorhexidine (CHX) into dental composite resins (DCR) offers antibacterial benefits but may affect their mechanical and esthetic properties. This in vitro study aimed to evaluate the impact of different CHX concentrations (0.12%–3%) on polymerization, hardness, bond strength, shade, and antibacterial activity.

Methods: A flowable composite resin was enriched with CHX at various concentrations. The degree of conversion (DC%) was measured using Fourier-transform infrared spectroscopy. Hardness was assessed with a Shore-D durometer, and shear bond strength (SBS) was tested on human dentin surfaces. Shade variations were evaluated using spectrophotometry. Antibacterial efficacy was determined by measuring inhibition halos against Streptococcus mutans (S. mutans), Lactobacillus fermentum and Lactobacillus paracasei. Scanning electron microscopy (SEM) analyzed CHX distribution in the resin matrix.

Results: CHX up to 1.5% did not significantly alter DC%, but the concentration of 3% impaired polymerization (p = 0.049 vs. control). Both hardness and SBS decreased significantly compared to the control group from concentrations as low as 0.2% for hardness and 0.12% for SBS (p < 0.001 in both cases). Notably, SBS values fell below 5 MPa at a concentration of 1% CHX. Shade modifications exceeded the perceptibility threshold, with CHX-containing composites appearing lighter. SEM images revealed inhomogeneous CHX distribution and microgaps in the resin matrix. Antibacterial tests demonstrated a dose-dependent inhibition of bacterial growth, with significant effects at 0.5%–1% CHX.

Conclusion: While CHX incorporation enhances antibacterial properties, concentrations above 0.5% compromise mechanical properties and esthetics, limiting clinical applicability. Future research should explore strategies to optimize CHX release and maintain mechanical integrity, such as hybrid antibacterial agents or controlled-release systems, to balance antibacterial efficacy with structural stability.