Substance use disorders are conditions where use of alcohol, drugs or other substances continues despite causing harm to health, work, relationships or daily life.
They are diagnosed by the pattern of use and its effects, such as needing more for the same effect, being unable to reduce use and giving up other activities. They range from mild to severe.
Adults with ADHD are more likely than others to develop a substance use disorder. For some, alcohol, cannabis or nicotine started as a way to quieten constant thoughts, sleep or cope with social situations. That's not an excuse. It's useful information for an assessment.
Stimulant medication needs careful planning if there is a current or past substance use disorder, so tell the clinician. It affects which medication is safe to consider. It does not stop you having an assessment.