Since our previous research established that tramadol induces endoplasmic reticulum stress (ER stress) through the p-eIF2α (phosphorylation of the eukaryotic initiation factor 2)/ATF4 (activating transcription factor 4)/CHOP (C/EBP homologous protein) signaling axis in breast cancer cells, we aimed to investigate whether O-desmethyltramadol, the active metabolite of tramadol, also triggers ER stress in these cells. The gene discussed is ATF4; the disease is breast cancer.