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The combination of mental illness and substance abuse can be difficult to diagnose. Many of the signs and symptoms of substance abuse, including rebellious or argumentative behavior, also commonly occur in patients with mental health disorders who do not abuse substances. In addition, many signs and symptoms of substance abuse (e.g., paranoia) may be similar to those of certain mental illnesses.

It can be difficult and time-consuming for mental health professionals to determine which symptoms are due to substance abuse, which occur because of mental illness, and which are the result of the interaction between the two. Signs that may indicate that a patient with a mental disorder is abusing a substance may include:

• Sudden money problems
• Sudden change in friends
• Disappearance of valuables
• Appearance of drug-related items
• Extended periods in private, such as in the bathroom
• Dilated or pinpoint pupils
• Needle marks on the body