Actually biochemical. A biochemical imbalance is either too much of a neurotransmitter (e.g., Serotonin, dopamine, norpinephrine), not enough of a neurotransmitter, or problems with neuroreceptors (desensitization). These transmitters and receptors are used to carry the information over the synaptic gap (the space between neurones). Many drugs artificially raise the amount of particular neurotransmitters, mimic them, or alter the receptors (e.g., nicotine alters ACh receptors).
A person with a biochemical imbalance could therefore being using certain drugs to bring the balance between receptors and transmitters to normal. Self medicating.
That said, I think the vast majority of addicts are not self medicating. I think the vast majority just like euphoria. But you don't know until you look.
