This randomized, double-blinded, placebo-controlled clinical trial aims to evaluate the efficacy and safety of naltrexone in reducing nonsuicidal self-injurious behavior among individuals with nonsuicidal self-injury. Participants will be randomly assigned to receive either naltrexone plus treatment as usual or placebo plus treatment as usual for 6 weeks.

The primary objective is to determine whether naltrexone reduces the frequency of nonsuicidal self-injurious behavior compared with placebo. Secondary objectives include evaluating changes in clinical severity, suicidal ideation, self-injury-related urges, ecological momentary assessment measures, and safety outcomes.

Nonsuicidal self-injury is defined as the intentional destruction of one's own body tissue without suicidal intent. It is clinically important because it is associated with emotional dysregulation, impulsivity, psychiatric comorbidity, and increased risk of future suicidal behavior. However, evidence-based pharmacological treatments specifically targeting nonsuicidal self-injurious behavior remain limited.

Naltrexone is an opioid receptor antagonist that has been suggested to reduce repetitive self-injurious behaviors by modulating endogenous opioid-related reinforcement mechanisms. This study will investigate whether naltrexone is effective in reducing nonsuicidal self-injurious behavior in a randomized, double-blinded, placebo-controlled design.

A total of 150 participants will be enrolled across multiple study sites. Eligible participants will be randomly assigned in a 1:1 ratio to either the naltrexone group or the placebo group. The intervention period will last 6 weeks, with clinical evaluations conducted at baseline and every 2 weeks thereafter. Smartphone-based ecological momentary assessment will also be used to monitor self-injurious urges, mood states, and related behavioral variables during the study period.

Safety will be assessed throughout the study through adverse event monitoring, assessment of suicidal ideation and behavior, laboratory testing, urine testing, and electrocardiography according to the study schedule.

Inclusion Criteria:

• Participants must meet all of the following criteria:

  1. Individuals aged 16 years or older.
  2. Individuals with clinically significant nonsuicidal self-injurious behavior.
  3. Individuals who are able to understand the study procedures and provide written informed consent. For minors, consent from a legal guardian and assent from the participant will be obtained according to applicable regulations.
  4. Individuals who are able to comply with study procedures, including clinical visits, medication administration, and study assessments.
  5. Women of childbearing potential must have a negative urine pregnancy test at screening and agree to use appropriate contraception during the study period.

Exclusion Criteria:

• Participants meeting any of the following criteria will be excluded:

  1. Current serious suicidal ideation or high suicide risk, as determined by the investigator.
  2. Current opioid use, opioid dependence, or use of opioid-containing medications.
  3. Current use of opioid antagonists or medications that may interact with naltrexone, including methadone or buprenorphine.
  4. Use of naltrexone within 1 week before screening.
  5. Positive naloxone challenge test or positive urine opioid test, if applicable.
  6. Known hypersensitivity to naltrexone or any component of the investigational product.
  7. Active liver disease, active hepatitis, or clinically significant hepatic impairment.
  8. Clinically significant renal impairment.
  9. Pregnancy or breastfeeding.
  10. Intellectual disability, organic brain disorder, or other condition that may interfere with the participant's ability to understand study procedures or complete assessments.
  11. Inability to read or write Korean sufficiently to complete study assessments.
  12. Documented prior non-response to naltrexone for nonsuicidal self-injury, as judged by the investigator.
  13. Any other clinically significant medical or psychiatric condition that, in the opinion of the investigator, would make participation inappropriate or unsafe.

Individual participant data will not be publicly shared due to privacy and ethical restrictions. De-identified data may be made available from the corresponding investigator upon reasonable request and with appropriate institutional approval.

Participants and clinical outcome assessors will be blinded to treatment allocation. Randomization and investigational product assignment will be managed separately by designated unblinded personnel who are not involved in clinical outcome assessments.