Underreporting of behavioral adverse effects is well known in patients undergoing chemotherapy for cancer. The behavioral Adverse Drug Reactions (ADR) could include delirium, insomnia, mood disturbances, or syndromal psychiatric diagnosis. Liaison with the psychiatry team is essential to address the evolution of new-onset behavioral symptoms and a high degree of sensitivity with good interdisciplinary coordination is expected during Pharmacovigilance of behavioral ADR. We report the emergence of Mania on Capecitabine and Oxaliplatin regimen in a case of carcinoma rectum. The possible role of the kynurenine pathway in the inflammatory cascade is hypothesized to be implicated in both cancer and mood disorders that require further research. A low dose of antipsychotics such as risperidone under supervised follow-up with psychiatrist is necessary to treat the organic mania. The report also highlights the importance of pharmacovigilance while reporting behavioral adverse effects in clinical practice and emphasizes the role of consultation-liaison psychiatry in the field of oncology.