Both alcoholism and the comorbid disorder may be caused, separately, by some third condition. Most studies of comorbidity are based on clinical samples. This trend may be countered to some extent by the reluctance of some alcoholism treatment centers to admit persons exhibiting serious psychiatric problems. Additional methodological difficulties complicate both clinical and general population investigations. For example, estimates of comorbidity will also vary depending on how alcohol use disorders are defined.
Similar considerations apply to the evaluation of comorbid disorders.
Moreover, the ECA program defined alcohol use disorders as the occurrence of enough symptoms to meet the associated diagnostic criteria over the life course. The sporadic occurrence of isolated symptoms, perhaps years apart, provides an insufficient basis for testing competing hypotheses related to comorbidity. A discussion of comorbidity with psychiatric disorders will follow. Alcohol has been shown to be directly toxic to the liver. Alcohol can damage the brain in many ways.