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.
Physical dependence is another quality of drugs that produce addiction.
A person who has taken a drug long enough to develop a tolerance will experience withdrawal symptoms when the drug is taken away. Withdrawal symptoms vary depending on the type of drug the person has been using, but these symptoms are all changes in physiology that can be measured. The last characteristic of drugs that produce addiction is a craving for the substance. A craving is a compulsive desire for the drug even if there are no withdrawal symptoms. A long period of time may have passed since the person has had access to the drug, but in certain situations and under certain circumstances the person has an overwhelming desire to use the drug again.