PGP and cancer: Frail patients with cancer treated with either VKA or DOAC are highly likely to experience pharmacokinetic drug–drug interactions due to the need for concomitant antineoplastic drugs, drugs to treat comorbidities, drugs to relieve symptoms from cancer, and drugs to treat cancer treatment-induced adverse events which are frequently either P-gp and CYP 3A4 inhibitors or inducers.