In this paper, we introduce and theorize the concept of toxic experts as individuals who, by virtue of their perceived or actual expertise, systematically engage in behaviors characterized by professional and intellectual vices. Despite maintaining an appearance of legitimacy, toxic experts exploit public trust by disseminating unsubstantiated, misleading, or harmful claims for personal and commercial gain. Drawing on a multidisciplinary framework, we integrate diverse insights to explain how toxic expertise emerges and persists. Specifically, we combine ethical and epistemic perspectives that distinguish genuine expertise from opportunistic misrepresentation. We analyze how social and institutional recognition shapes expert authority. Then we examine how structural transformations of work erode professional integrity and identify cognitive mechanisms that sustain trust in unverified claims. Using the case of the longevity biotechnology business, we develop a multilevel relational theoretical framework that identifies: (i) the historical and socio-cultural preconditions that enable toxic experts to emerge, (ii) the social and cognitive processes through which they gain and maintain legitimacy, and (iii) prevention strategies centered on cascaded accountability reforms. Our contextualized perspective challenges the depiction of toxic experts as isolated deviant individuals, revealing them instead as products of broader social, institutional, and ideological conditions. We argue that mitigating their influence requires cascaded regulatory interventions at societal and institutional levels to restore public trust and prevent toxic outcomes.