Final answer:
When substance levels in the body decline after prolonged heavy use, tolerance reverses, meaning less of the substance may be needed to achieve the same effects. However, this does not cure addiction and may result in withdrawal symptoms, which can be severe and require medical supervision.
Step-by-step explanation:
When blood or tissue concentrations of a substance decline in an individual who has maintained prolonged heavy use of the substance, tolerance reverses. Tolerance is defined as a condition that occurs when a drug user requires more and more of a given drug to experience the same effects of the drug. When the concentration of the drug in the body decreases, the individual's tolerance starts to diminish, meaning that if they were to take the drug again, they might require less of it to achieve the effects they had grown accustomed to while using heavily.
Additionally, the decrease in concentration does not mean addiction is cured; instead, the individual may experience withdrawal symptoms. Withdrawal from psychoactive drugs can involve a variety of negative symptoms which are typically opposite to the effects of the drug. Notably, because withdrawal can be a life-threatening process, particularly with substances like alcohol and barbiturates, it is important for those undergoing withdrawal to do so under medical supervision.