True to its name, obsessive-compulsive disorder (OCD) involves both recurring obsessions and compulsions. Obsessions are intrusive thoughts, while compulsions are rigid and repetitive behaviors (e.g., handwashing) or mental acts (e.g., counting) that are excessive or aimed at preventing an unwanted event in an unrealistic way. Previously categorized as an anxiety disorder, it’s not uncommon for OCD to co-occur with substance use disorders. Similarly, post-traumatic stress disorder (PTSD), which is now categorized separately from anxiety and obsessive-compulsive disorders, also frequently co-occurs with substance use disorder.