However, there is a problem with this explanation. The pharmacological action of the drug does not explain why people’s behaviour changes in the long term. It is presumed that once exposed to high levels of the drug, receptor density or activity changes. To maintain the same level of neuronal activity, the person must maintain high concentrations of the drug in the brain. Without this, the user becomes depressed, suffers discomfort, and seeks the drug to restore “normal” level of neuronal activity and a tolerable subjective state of well being. Those who become “addicted” have genetically-determined receptor deficits that make them chemically stimulate themselves to medicate their “inadequate” receptor activity.