The β2‑adrenergic receptor (β2‑AR, encoded by the ADRB2 gene) is a member of the G‑protein‑coupled receptor superfamily that can be stimulated by catecholamines. Studies in vivo and in vitro have confirmed that β‑blockers (β‑AR antagonists) exert antitumor effects on various tumors. Furthermore, ADRB2 single‑nucleotide polymorphisms (SNPs) have been identified to alter the expression and conformation of β2‑AR, which may alter the β‑blocker drug response. The aim of the present study was to investigate the effect of β‑blockers on triple‑negative breast cancer cells and determine whether ADRB2 SNPs affect the response to β‑blocker drugs. Propranolol and ICI 118,551 significantly inhibited the viability of MDA‑MB‑231 cells, arrested cell cycle progression at G0/G1 and S phase and induced cell apoptosis. Western blot analysis indicated that the phosphorylation levels of extracellular‑signal‑regulated kinase (ERK)1/2 and the expression levels of cyclo‑oxygenase 2 (COX‑2) were significantly decreased following β‑blocker treatment. Four haplotypes, which comprised ADRB2 SNPs rs1042713 and rs1042714, were transfected into 293 cells. After 24 and 48 h of transfection, ADRB2 mRNA expression was significantly decreased in mutant groups compared with the wild‑type group. The ADRB2 SNPs exerted no effect on cell viability, but did affect the drug response of ICI 118,551. Furthermore, ADRB2 SNPs also affected the regulatory function of ICI 118,551 on the ERK/COX‑2 signaling pathway. Collectively, propranolol and ICI 118,551 inhibited the viability of MDA‑MB‑231 cells by downregulating the ERK/COX‑2 signaling pathway and inducing apoptosis. The results of the present study indicated that SNPs rs1042713 and rs1042714 of ADRB2 affected the response to ICI 118,551, and the underlying molecular mechanism was elucidated.