Both fentanyl and heroin are considered to be extremely addictive drugs. They can produce an intense and euphoric “high,†and dependence can form quickly. Once the brain becomes used to the presence of an opioid drug, the body will require the drug in order to function optimally. Significant withdrawal symptoms can occur when the drug wears off. These effects include depression, anxiety, irritability, agitation, insomnia, and physical symptoms similar to those experienced with the flu. It may seem better to just keep taking the drug than deal with these painful withdrawal symptoms, and this can quickly lead to an inability to control how much and how often the drug is taken. Compulsive drug use caused by altered brain chemistry as the result of regular drug abuse is the definition of addiction.