TEVROMATIN:

PROFILE: Chemotherapy adjuvant specifically designed for glioblastomas of neuronal origin. By mimicking natural neural differentiation factors, it causes these tumors to regress from resilient high-grade neuroblasts towards more typical neurons, making them easy targets for stronger chemotherapeutic agents.

BANNED BECAUSE: During differentiation process, malignant nerve cells form connections to healthy nerve cells and to each other. As a result, tumor forms a functioning neural network effectively “telepathically” connected to healthy brain. Patients report feelings of overwhelming guilt as tumor accesses patient’s memories and emotions and realizes its role as a parasitic cancer, followed by its utter terror as it realizes it is about to be killed. Many patients refuse to continue with chemotherapy regimen; those who continue make a complete physical recovery but are psychologically scarred for life as they experience every moment of the tumor’s death as if it were their own.

My favorite item in the Yvain's list of fictional banned drugs.