Inclusion criteria:
Diagnosis of moderate to severe opiate use disorder including only opium, Shireh (opium concentrate), heroin, and/or heroin crack based on DSM 5
Voluntarily seeking treatment and willingness to receive antagonist maintenance treatment
Successful completion of medically-assisted opioid withdrawal (including a personal report of at least 7 days of abstinence from opioids, negative urinalysis test for morphine, methadone, buprenorphine and tramadol, and negative antagonist challenge test)
Good physical health based on medical records, physical examination and measurement of vital signs
Hematology tests, liver and kidney function tests in the normal range
In women of childbearing age agree to receive a safe method of contraception
Living in Tehran
Having a family member or companion who can monitor regular visits and daily use of the study medication
No severe abnormalities in the ECG
Ability to understand and give written informed consent to enter the study
Exclusion criteria:
Positive antagonist challenge test
Positive urinalysis test for opioids (morphine) or opioid drugs (methadone, buprenorphine, tramadol)
Patients undergoing maintenance treatment with opioid drugs (methadone, buprenorphine, or opium tincture)
Request treatment and desire to receive opioids maintenance treatments with medication like methadone and buprenorphine
People treated with opioid antagonists or a history of continuous naltrexone injections in the past 6 months
Need to receive opioid medications for medical purposes such as chronic pain disorder
Concurrent diagnosis of other substance use disorders such as stimulants and sedatives (except nicotine and caffeine)
Concurrent alcohol use disorder and people who are currently need medical management for alcohol withdrawal
Pregnancy, lactation or women of childbearing age who are planning to become pregnant or do not use a contraceptive method
Having uncontrolled, major psychiatric disorders at the time of referral (e.g. mood disorders with impaired function, schizophrenia, etc.) that can interfere with participation in the study
Severe and uncontrolled physical conditions based on physical examination by study's physician and laboratory evaluations (e.g. uncontrolled hypertension or diabetes mellitus)
Having uncontrolled medical illness that can interfere with study or significantly increase medical risk. (Such as high blood pressure and uncontrolled diabetes)
Severe physical illness (including acute renal failure, endocarditis and tuberculosis)
Liver failure, acute hepatitis or more than threefold increase in liver enzymes
Thrombocytopenia and a history of coagulation disorders
Planning for surgery during the study period
Inadequate physical condition for safe intramuscular injection of extended-release naltrexone, body mass index above 40 (excessive adipose tissue in the buttocks) or severe weight loss
The presence of AIDS-indicator disease
Known sensitivity to naltrexone, polylactide-co-glycolide, arboxymethylcellulose, or other components of the effective and soluble extended-release injectable naltrexone
Serious suicidality or a history of suicide last year
Last year history of drug overdose that led to hospitalization
Being on probation or under judicial supervision with the possibility of re-arrest and re-imprisonment
Inability to understand the study protocol or respond to evaluations as well as lack of proper communication with therapists