People interested in drug addiction should become more interested in the processes of learning and memory. Numerous studies have shown that there exists a direct link between drug addiction and the learning and memory processes.
Addictive drugs have three defining characteristics.
The first is tolerance. Tolerance is the condition where the effect of the drug decreases after repeated administrations. For example, the first time morphine is administered for pain relief it is very effective. However, after repeated dosages, the effectiveness of morphine gradually decreases. Tolerance, then, is the decreasing efficiency of a drug due to repeated administrations.