Polypharmacy and drug-drug interactions in metastatic breast cancer patients receiving cyclin-dependent kinase (CDK) 4/6 inhibitors.

If CDK 4/6 inhibitors interact with concomitant drugs, they may cause an increase in the incidence of cardiac side effects and a decrease in the effect of the CDK 4/6 inhibitor or additional drug or an increase in toxicity. Increasing awareness of this issue will help to reduce the rates of side effects or toxicity and provide effective antitumour therapy.