Substance abuse can lead to addiction or dependence. When addicted to a substance, the user has to depend on it for his daily activities. He / She would no longer be in control of his own life but becomes obsessed with the next fix. If he does not have the money to finance the addiction, he is likely to lie, cheat or steal to obtain money to buy substances. This will lead to many other problems.

Substance addiction or dependence may take two forms:

Physical Dependence
Drug abusers are compelled to take the substance regularly or they would suffer from tremendous pain and discomfort.
Symptoms such as restlessness, runny nose, and abdominal cramps may show depending on the substance consumed.

Psychological Dependence
Addiction also causes a psychological craving for the substance that will cause the user to do almost anything to obtain the next fix. It is more difficult to cure this form of dependence and many ex-addicts continue to take drugs even after they are no longer physically dependent on them.

TOLERANCE
Addiction can also lead to an increased tolerance of the substances. This simply means that a person who abuses substances for pleasure regularly has to gradually increase the dosage to get the same effect (to get high).

If the person stops taking the substance for some time, he will lose tolerance of the substance. Subsequently if he/she increases the usual dosage, death from overdose may result.