Addiction is defined as a chronic, relapsing brain disease that is characterized by compulsive drug seeking and use, despite harmful consequences.
Dependence develops when the neurons adapt to the repeated drug exposure and only function normally in the presence of the drug. When the drug is withdrawn, several physiological reactions occur (known as withdrawal syndrome). http://www.drugabuse.gov/pubs/teaching/Teaching2/Teaching4.html
Tolerance occurs when the person no longer responds to the drug in the way that the person initially responded. Stated another way, it takes a higher dose of the drug to achieve the same level of response achieved initially.
Do you use drugs or drink alcohol? Are these substances harming your health or increasing your risk for other problems? Take a self-assessment to determine if you have issues with substance abuse.