Introduction: Folie à Deux (Shared Psychosis) is a rare psychiatric phenomenon in which delusional beliefs are transmitted from a dominant primary patient to a submissive secondary patient, typically within insular dyadic relationships.

Background: Sisters A (43F) and B (48F) lived in a socially isolated household with their mother. Eighteen months prior to presentation, Sister A developed persecutory delusions surrounding a perceived "cyber-conspiracy," triggered by recurrent transaction failures and phone malfunctions. Sister B developed congruent but less firmly held delusions six months later. Both exhibited prominent obsessive-compulsive (OC) traits ; religious ritualism and compulsive hoarding of packaged water bottles, the latter precipitating a house fire. Their severe social withdrawal proved fatal for their mother, who deteriorated from food and water deprivation, while neither sister sought help. Her death became a significant psychosocial stressor, markedly exacerbating Sister B's symptoms.

Management: On admission, A was diagnosed with Paranoid Schizophrenia (PANSS: 96) and B with Shared Delusion: Folie Imposée (PANSS: 68). After separation and antipsychotic initiation, both responded well — A to Amisulpride 600 mg, B to 300 mg — with marked improvement at two months (PANSS: A-55, B-36). OC symptoms subsequently emerged, paradoxically more severe in B (Y-BOCS: B-23, A-12) with poorer insight. Fluoxetine was added (A-40 mg, B-60 mg), Amisulpride tapered to 100 mg, and CBT initiated.

Discussion & Conclusion: Paradoxically, the dominant primary patient showed greater psychotic conviction yet milder OC symptoms and better insight, while the submissive secondary patient bore a greater OC burden with poorer insight. This case highlights the critical importance of temporal reassessment and diagnostic flexibility in rare psychiatric presentations.

Purbasha Sengupta completed her MBBS from Institute of Post-Graduate Medical Education & Research (IPGMER), Kolkata. She is currently in her final year of postgraduate training in Psychiatry at the Centre of Excellence, Department of Psychiatry, Calcutta National Medical College, Kolkata. She has 4 publications to her credit and has recently completed her dissertation examining the relationship between lipid biomarkers and schizophrenia. She is an active member of the Indian Psychiatric Society. During her residency, she has participated in over 10 national & state conferences and 3 workshops, demonstrating a strong commitment to academic and clinical psychiatry. Notably, she has co-organised a workshop on Transactional Analysis in collaboration with the IPS-State Branch, and attended an exclusive neuropsychiatric intervention observership at the Central Institute of Psychiatry (CIP), Ranchi, reflecting her dedication to advancing her clinical expertise beyond the conventional training curriculum.