The relationships between these disorders are complex and not fully understood. Drug abuse can mask, mimic, or result from a wide range of psychiatric and medical disorders. Drug abuse can also exacerbate an existing psychiatric disorder such as the manic phase of bipolar disorder or be exacerbated by a developing psychiatric disorder. The questions of cause and effect are an important unresolved issue; much research support exists for both theories of causal directionality, and some researchers believe that both types of disorders are the common result of a common genetic predisposition. Nevertheless, both drug abuse and depression are underrecognized and undertreated. Many individuals with one or both disorders do not seek professional treatment. When they do, the depression or the drug abuse is sometimes missed. In addition, the general physicians who are more likely to be consulted initially than psychiatrists often prescribe inadequate and ineffective pharmacological treatment. Patient denial or dissembling, inadequate history gathering by the physician, or the physician's unfamiliarity with the treatment continuum may cause the physician to overlook the need for specialized intensive treatment for drug abuse. Treatment programs are often designed only for one category of disorder, and staff may be unequipped to deal with the other type of disorder. Therefore, health professionals need to understand, assess, and treat both disorders. Tables and 13 references