A substance use disorder, such as heroin use disorder, is a condition where a person will have great difficulty ending their substance use despite the negative impact it’s having on their life. Someone who has a substance use disorder will build a tolerance to the substance, meaning they will need more of the substance to get the same results. Also, substance use disorder affects how the brain works. It can contribute to mental illness and negatively affect cognitive function. The good news is that the brain can heal when substance use ends and many people recover their brain function.