A 32-year-old man suffering from paranoid schizophrenia was referred for excessive weight gain attributed to risperidone, resistant to a dietary regimen. The patient had a first episode with auditory hallucinations and persecutory delusions 8 years ago. He had been medicated with risperidone, and symptoms were well-controlled. After an unsuccessful reduction of risperidone, the patient was stabilized on 4 mg/day. A gradual switch to paliperidone 9 mg/day was tried. Positive symptoms, that is, prominent hallucinations or formal thought disorder, were adequately contained, but, after 2 months, the patient complained of uncontrollable, invasive thoughts of harming his parents, which had never been present beforea. The patient recognized the irrational nature of his thoughts, and these produced excessive anxiety that he could not manage. A switch back to risperidone led to full remission of obsessive thoughts in 2 weeks.