OBJECTIVESTo examine how frontline health care workers (HCWs) and decision-makers working in tuberculosis (TB) care and mental health care conceptualise liberty, harm and the harm principle-which are often invoked in health care legislation to justify liberty restrictions-in the treatment and care of persons with TB and severe and persistent mental illness (SPMI). DESIGNQualitative study of 20 semi-structured interviews with HCWs and decision-makers working in public or mental health from three public health units and two psychiatric hospitals in the Greater Toronto Area in Ontario, Canada. Interviews were evaluated using thematic analysis. RESULTSThree themes were identified: 1) the contextual nature of liberty restrictions; 2) the concept of liberty as a matter of degree; and 3) the challenges of balancing the treatment and care goals of TB and mental health for persons with both TB and SPMI. CONCLUSIONHarm is understood by HCWs caring for persons with TB and/or SPMI in a context-dependent manner. This is compounded in the case of persons with both TB and SPMI. Liberty restrictions to advance public health goals entail reciprocal obligations from society, including social protections and additional resources, to ease the effects and range of liberty restrictions. Attention is required to ensure that treatment and care for SPMI does not impede that of TB and vice versa.