耐药性或抗药性（drug resistance）是指药物的治疗疾病或改善病人征状的效力降低。当投入药物浓度不足，不能杀死或抑制病原时，残留的细菌可能具有抵抗此种药物的能力。例如细菌可能因抗生素产生的活性氧诱发DNA突变而造成耐药性。这种现象被称为hypermutability。

耐药性一词等于药物剂量失败或药物耐受。耐药性多指由病原体引起的疾病，而耐药性则亦指因长期服药，造成相同剂量却不如当初有效的情况。耐药性产生亦可能是抗生素的滥用，或未按处方服药。

Drug resistance is the reduction in effectiveness of a drug such as an antimicrobial, anthelmintic or an antineoplastic in curing a disease or condition. When the drug is not intended to kill or inhibit a pathogen, then the term is equivalent to dosage failure or drug tolerance. More commonly, the term is used in the context of resistance that pathogens have "acquired", that is, resistance has evolved. Antimicrobial resistance and antineoplastic resistance challenge clinical care and drive research. When an organism is resistant to more than one drug, it is said to be multidrug-resistant. Even the immune system of an organism is in essence a drug delivery system, albeit endogenous, and faces the same arms race problems as external drug delivery.