What is the biological basis for drug dependency?
Dependence on a substance, whether legal or illegal, has very much to do with human biology. Our bodies and minds function thanks to lots of minute chemical processes taking place all throughout our many systems and tissues. Some people feel that the determining factor for legality of a substance, and therefore dependence upon it, is whether it impacts the brain or the body and in what way.
An example of a legal substance dependency is insulin use in people with diabetes. People with diabetes have a chronic deficiency or inability in their natural production of insulin. This is an important chemical in the body, as it helps us properly use or store sugars in the blood. Without insulin, people who have diabetes may suffer from fatigue, dehydration, dry skin, blurry vision, and even be at risk for coma. The drug insulin helps a diabetic person's body to function normally to metabolize the sugars in foods.
An illegal substance dependency is the dependency on a drug which has been criminalized, like heroin. Heroin is not used to treat a specific condition, though many people self-medicate conditions like anxiety, depression, and Post-Traumatic Stress Disorder by using mind-altering substances. When heroin is taken into the body, it binds to opioid receptors in the brain. This drug is highly addictive because it produces an overwhelming sense of euphoria and calm. Because the drug is so powerful and works so quickly, these opioid receptors in the brain can become "lazy" and fail to function normally when there is no heroin in a person's system. This can also contribute to an increased tolerance level, meaning it will take a person more heroin to achieve the same state of euphoria. Heroin poses a major risk for overdose because people are prone to taking more and more of the drug to keep feeling good.
In both cases, the substance dependency is preceded by a biological failure to function "normally" without exposure to the substance.