It’s not clear yet how the body responds to addictive substances. Whether there is a biologic or inherited factor remains uncertain. Some scientists think genetics combined with environmental interaction can actually trigger a substance abuse disorder at conception.

Studies in mice suggest there may be a genetic link. Certain strains of mice seem to have a greater risk for addiction when given the same drug type and dosage. Other studies have looked at the possibility that pain (for which the drug is taken) changes the way the drug works.

There's further evidence to suggest that pain and genetics interact to change the effects of substances like morphine. Perhaps some individuals have a greater sensitivity to opioids, and others are resistant to their effects. Much more study is needed to sort this all out and find ways to separate the mind altering from pain-relieving qualities of opioids.