This case shows that even fenotiazine derivatives, which are believed to be relatively safe because of their wide therapeutic index, may cause fatal intoxication if administered at a high dose. In the case at issue, the administered dose of promazine was determined to be over 100 tablets, 100 mg each. Immunological methods (EMIT, FPIA) were used to check for the presence of ethanol, benzodiazepines, barbiturates, and tricyclic antidepressants in the blood and urine. No ethanol was found in the blood sample; and no benzodiazepines, barbiturates, or tricyclic antidepressants were detected in the urine. The only detected compound was a fenotiazine derivative in blood and urine. This was determined with the use of high-performance liquid chromatography with Diode Array Detector. The determined blood concentration of promazine (8.47 mg/l) was higher than the lowest lethal concentration of promazine described in the literature (5 mg/l). The patient was found unconscious in the morning hours with some partly emptied medicine packaging nearby. She was hospitalized in severe condition. This paper describes her symptoms and the treatment she received in the hospital until her death 19 hours after hospitalization. The patient had been under treatment after several previous suicide attempts. 2 tables, 2 figures, and 8 references