Research has shown that people with an opioid use disorder are at higher risk for other co-occurring psychiatric disorders (called “psychiatric comorbidities”). About half of those who experience a mental illness during their lives will also experience a substance use disorder and vice versa. 
Psychiatric disorders include depression, anxiety, attention-deficit hyperactivity disorder (ADHD), bipolar disorder, psychotic disorders and/or post-traumatic stress disorder (PTSD). Other substance use disorders (SUDs) such as abuse of nicotine, alcohol, cannabis, benzodiazepines, and/or stimulants can also be co-occurring in opioid use disorders as well. Though it is common for SUDs and mental illness to present together, it can be difficult to assess which came first- the substance use or the mental illness. It is important to ensure that those with an opioid use disorder have an appropriate evaluation by a mental healthcare provider to identify any comorbidities that may underly the substance use.
Substance-induced psychiatric disorders can occur when someone who uses opioids (or other drugs of abuse) experience psychiatric symptoms only while using a drug or when in withdrawal from a drug. Many opioid users experience lower motivation, depression, irritability, difficulty with sleeping too much or too little and behavioral changes and only occur when they are intoxicated or in withdrawal from opioids. Substance-induced psychiatric disorders fully resolve/recover once the drug use has stopped for a period of time (abstinence). However, with opioid abuse, withdrawal can be excruciating without appropriate treatment and often results in multiple relapses over time. Ongoing substance use may result in brain changes that may eventually lead to the development of a mental disorder.
When psychiatric symptoms persist despite abstinence from the drug use, it is important to consider that there may be a psychiatric disorder co-occurring and independent of the person’s substance use. Symptoms of a psychiatric diagnosis can present at any time, however in order to make a formal diagnosis, the individual must have symptoms before beginning opioids and/or after cessation of opioids and completing opioid withdrawal. When a psychiatric disorder is made, it is important to understand that some people may use substances to self-medicate the symptoms of their mental disorder. The substance use may provide some temporary relief at first, however due to the addictive nature of many substances, drug abuse often makes psychiatric symptoms worse in the long run. Common risk factors for development of a psychiatric illness and/or SUD include genetics that are passed down from generation to generation and environmental influences, such as trauma and/or stress.
It is important to discuss the lethality associated with co-occurring mental illness and SUDs, especially with the recent increase in opioid-related overdose deaths. People who take high doses of opioids are at increased risk of unintentional overdose. It has also been reported that people with a prescription opioid use disorder are twice as likely to attempt suicide than those who do not misuse prescription opioids.  For those who have suffered the loss of a loved one by opioid-related overdose, it can be difficult and challenging to determine if there was an underlying intent to commit suicide. Some experts approximate that up to 30% of opioid overdoses may fit the description of suicide. 
When seeking treatment, people with co-occurring psychiatric disorders may find recovery more difficult and may require intense treatment. The most effective treatments address substance use disorders and all other active psychiatric disorders together, instead of separately.
 NIDA. 2021, April 13. Part 1: The Connection Between Substance Use Disorders and Mental Illness. Retrieved from https://www.drugabuse.gov/publications/research-reports/common-comorbidities-substance-use-disorders/part-1-connection-between-substance-use-disorders-mental-illness on 2021, May 10
 NIDA. 2019, September 19. Suicide Deaths Are a Major Component of the Opioid Crisis that Must Be Addressed. Retrieved from https://www.drugabuse.gov/about-nida/noras-blog/2019/09/suicide-deaths-are-major-component-opioid-crisis-must-be-addressed on 2021, May 10