 Drug use problems are often associated with health harms. Infectious diseases such as HIV, AIDS and hepatitis C have clear links to drug use. But there are also links between drug use and mental health problems that we need to consider. People with problematic forms of drug use often suffer from both a substance use and a mental health disorder. The coexistence of a substance use and a mental health disorder in the same person is usually called psychiatric comorbidity in substance users or dual diagnosis. There aren't many figures on the extent of mental health disorders among drug users, but the numbers affected can be very high, up to 80% in certain groups of drug treatment patients. Depression is the most common mental health disorder associated with substance use, but anxiety is also often seen and comorbid substance use disorders are more common in people with psychosis. Compared with people who have only a drug use or a mental health problem, people with comorbidity have more emergency admissions to general hospitals and to psychiatric hospitals and more of them commit suicide. Substance use and mental health disorders are interlinked in different ways, which are sometimes overlapping. In some cases, a mental health disorder can be considered a risk factor for drug use, which may lead to the development of a substance use disorder. In other cases, drug use can trigger the development of a mental health disorder. Clinical practice has also shown that comorbid disorders often interact to make each other worse. It is often difficult to diagnose comorbidity among drug users. For example, in a person entering drug treatment, acute psychiatric symptoms might be attributed to their drug use, or the effects of withdrawal or intoxication from drugs may be misinterpreted as psychiatric illness. Patients need careful diagnosis and integrated treatment, where the two disorders are treated at the same time with professionals from both fields working together. Providing this level of care is a major challenge for policymakers, professionals and clinicians.