Carbamazepine (CBZ) is an anticonvulsant medication with highly persistent properties in the aquatic environment, where it has the potential to affect non-target biota. Since CBZ and many other pharmaceuticals are not readily removed in conventional sewage treatment plants (STP), additional STP effluent treatment technologies are being evaluated and implemented. Whole effluent ozonation is a prospective method to remove pharmaceuticals such as CBZ, yet knowledge on the toxicity of CBZ ozonation by-products (OBPs) is lacking. This study presents, for the first time, individual and mixture toxicity of four putative OBPs, i.e. carbamazepine 10,11-epoxide (CBZ-EP), 10,11 dihydro-carbamazepine (DI-CBZ), 1-(2-benzaldehyde)-4-hydro-(1,3)-quinazoline-2-one (BQM), and 1-(2-benzaldehyde)-(1,3)-quinazoline-2,4-dione (BQD) in developing zebrafish () embryos. BQM and BQD were isolated from the ozonated solution as they were not commercially available. The study confirmed that the OBP mixture caused embryotoxic responses comparable to that of ozonated CBZ. Individual compound embryotoxicity assessment further revealed that BQM and BQD were the drivers of embryotoxicity. OBP chemical stability in ozonated CBZ water solution during two-week dark storage at 22°C was also assessed. The OBP concentrations remained over time, except for BQD which decreased by 94%. Meanwhile, ozonated CBZ persistently induced embryotoxicity over two-week storage, potentially illustrating environmental concern.