Considerable comorbidity has been reported for alcoholism and other Axis I disorders in individuals who abuse drugs, and a relationship has been found between the type of comorbid disorder (eg, alcoholism or affective disorder) in probands and the rate of that same disorder in relatives. Relatives of opiate-dependent probands are at significantly increased risk for substance abuse, alcoholism, antisocial personality disorder, and major depression, compared to control subjects.
A study from a sample of 32 MZ twins raised apart found significant heritability for drug abuse or dependence, with a probandwise concordance rate of 36%. Concordance in this range suggests a combination of genetic and environmental effects.
Of 443 adoptees included in an adoption study of drug abuse, half were selected for psychopathology in biological parents and the other half were matched for age and sex. The parents were not examined directly, but information from adoption records was available. Of the adoptees, 40 manifested drug abuse of one kind or another. Antisocial behavior in a biological relative predicted drug abuse in the adoptee, and alcohol problems also predicted drug abuse. Possible environmental factors included divorce and significant psychiatric pathology in the adoptive parents.
Candidate Gene Studies
Variants of the sigma opiate receptor gene are being studied in a mouse model and in humans who abuse opiates. Initial results are promising.