2.1

Study design and setting

This prospective observational study evaluated patients with drug overdose between January 1, 2015, and July 30, 2021. The patients were transferred to the Japanese Tertiary Critical Care Center at the Tokyo Medical and Dental University Hospital of Medicine. This study complied with the principles embodied in the 1964 Declaration of Helsinki and its amendments. The study protocol was approved by the ethics committees of the Tokyo Medical and Dental University Hospital of Medicine (M2021–107). All patient data were retrospectively collected from electronic medical charts and anonymized before statistical analysis.

2.2

Study population

This study included consecutive patients who were transferred to the Tokyo Medical and Dental University Hospital of Medicine because of drug overdose. We excluded patients who met at least one of the following criteria: (1) age < 18 years, (2) <1 year of observation period after discharge, (3) inability to undergo follow-up, and (4) difficulty in collecting sufficient information by telephone interview.

In addition, in this study, patients aged >65 years were excluded since mental disorders in geriatric patients could often go along with other medical diseases and disabilities, such as dementia and physical illnesses [ ].

2.3

Data collection

Throughout the Institutional Review Board (IRB) application process, we engaged the expertise of two experienced psychiatrists to ensure that all the procedures were in compliance with ethical guidelines. We were mindful of the potential discomfort our patients might experience during this study. Therefore, we made it explicitly clear that the patients had the full right to withhold responses should they feel uncomfortable at any point during the telephone interviews.

These interviews were conducted exclusively for post-discharge consultation purposes, providing a focused scope for our interactions with the patients. We, as the primary interviewers, took conscious steps to respect our patients’ time and limit potential discomfort. We aimed to keep each interview as efficient as possible, making every effort to conclude them within a 10-min timeframe.

Patients’ medical records were searched to collect data on age, sex, medical history, length of hospital stay, number of ingested pills, type of medication used for the overdose, starting time of overdose, Glasgow Coma Scale score on arrival, history of psychiatric visits, presence of cohabitants, history of self-harm, and situation at the time of overdose. Overdoses without suicidal intent were excluded based on medical record information obtained from the patients and/or their caregivers in the first episode. In recurrent episodes, we collected this information by telephone interview with the patients or their caregivers. The psychiatric diagnosis was made by our psychiatrist based on ICD-10, or if the patient had a history of psychiatric problems, we requested information from the previous physician and obtained medical information to confirm the diagnosis based on ICD-10. Patients who were transferred before admission or those who were not followed-up after discharge by medical records were followed-up by telephone interviews to collect information on relapse within 1 year and progress after discharge.

2.4

Definitions and outcomes

The rate of recurrence of overdose was designated as the primary outcome. The recurrence interval, which was the time elapsed from the day of discharge for the first overdose admission to the day of the recurrent overdose, was designated as the secondary outcome. Recurrence of overdose was defined as a repeated episode of deliberate drug overdose within 1 year of discharge after admission for overdose [ , ]. The patients whose observation period after discharge from the hospital exceeded one year were included. The length of hospital stay was defined as the number of days of hospitalization at our hospital. Therefore, if the patient was transferred to another hospital on the day of transport, the length was hospital stay was assumed to be 1 day. Drug poisoning at hospital admission was defined as an overdose according to the International Classification of Diseases-10 codes T360-T509 [ ].

2.5

Statistical analysis

Categorical variables were reported as numbers (percentage), while continuous variables were reported as mean (standard deviation) or median (interquartile range), as appropriate. The normality of distribution of continuous variables was tested using the Kolmogorov-Smirnov test. We divided patients into the recurrence and non-recurrence groups, compared their clinical characteristics, and retrospectively examined the factors associated with recurrence. Multivariable logistic regression analysis was performed to assess the risk of recurrence of drug overdose. The variables incorporated into the model included history of schizophrenia, presence of cohabitants, and psychiatric intervention during hospitalization. These variables were selected from a clinical perspective. Furthermore, we used the log-rank test to visualize the relationship between recurrence rates and recurrence intervals by comparing patients with and without cohabitants.

Statistical analyses were performed using the R software (version 4.2.2; R Foundation for Statistical Computing, Vienna, Austria) and GraphPad Prism 9 (GraphPad, San Diego, CA, USA). Statistical significance was set at p < 0.05.