Breast cancer is one of the most common malignancies affecting many women around the world. In the case of advanced ER+, PR+/ HER2- breast cancer, the first line of treatment is a CDK 4 inhibitor in combination with endocrine therapy. Endocrine therapy has been the standard protocol for many years, and the mechanism of action and side effects of these drugs are widely known. On the other hand, the drugs, palbociclib, ribociclib, and abemaciclib, have only recently been approved for clinical practice. In addition to the more commonly reported CDK 4 inhibitors side effects such as nausea, loss of appetite, vomiting, fatigue, anaemia, neutropenia, etc., they also cause more severe side effects such as cardiotoxicity. Cardiotoxicity negatively affects the CV prognosis and limits the therapeutic possibilities of oncological treatment. QT prolongation is particularly associated with ribociclib and is dose-dependent and can cause fatal TdP. The arrhythmia may clinically manifest as palpitations, an uncomfortable feeling of heartbeat. Recognition and treatment of cardiotoxicity early on improves prognosis and quality of life. Other side effects are an important factor that further affects the quality of life. PALOMA - 2 and - 3 studies and real-world data showed that the pain scale was lower in patients treated with CDK 4 inhibitors with endocrine therapy compared to endocrine treatment with placebo. They also described their quality of life during CDK 4 inhibitor therapy as good and satisfactory. In addition, CDK4 inhibitors have been studied in various studies and have been observed to prolong progression-free survival. Equally important, if not more important than this, is the fact that these drugs are well tolerated and that patients have a maintained quality of life. The influence of CDK 4 inhibitors on QT prolongation and the sensation of palpitations and the mechanism by which it acts on the above need to be further investigated.