Cataracts have a high incidence and prevalence rate worldwide, and they are the leading cause of blindness. Lens epithelial cell (LEC) apoptosis is often analysed in cataract research since it is the pathological basis of cataracts, except for congenital cataract. Chloride channels are present in ocular tissues, such as in trabecular cells, LECs and other cells. They serve an important role in apoptosis and participate in endoplasmic reticulum (ER) stress and oxidative stress. However, their role in the apoptosis of LECs has not been discussed. The present study examined the effects of the chloride channel blocker 5‑nitro‑2‑(3‑phenylpropylamino) benzoic acid (NPPB) in human LECs (HLECs) to elucidate the role of NPPB in HLECs and investigate the role and mechanism of chloride channels in cataract formation. HLECs were exposed to NPPB. Cell survival rate was evaluated using Cell Counting Kit‑8 assays. Oxidative stress was detected as reactive oxygen species (ROS) in cells by using a ROS assay kit. Apoptosis was examined by assessing mitochondrial membrane potential and using a JC‑1 assay kit, and western blot analysis was performed to measure the expression levels of mitochondrial‑dependent apoptosis pathway‑associated proteins. ER stress was evaluated by determining the intracellular calcium ion fluorescence intensity, and western blot analysis was performed to measure ER stress‑associated protein expression. The results revealed that NPPB treatment decreased the viability of HLECs and increased apoptosis. Additionally, NPPB increased intracellular ROS levels, as well as the number of JC‑1 monomers and the protein expression levels of B‑cell lymphoma‑2 (Bcl‑2)‑associated X and cleaved caspase‑3, and decreased Bcl‑2 protein expression. NPPB increased intracellular calcium ions, the protein expression levels of activating transcription factor 6, JNK, C/EBP homologous protein and caspase‑12, and the phosphorylation of protein kinase R‑like endoplasmic reticulum kinase. N‑acetylcysteine and 4‑phenylbutyric acid inhibited NPPB‑induced oxidative stress, ER stress and apoptosis. Therefore, NPPB treatment decreased cell viability and promoted apoptosis of HLECs via the promotion of oxidative and ER stress.