Tolerance to a drug is the body's increasing resistance to the chemical's effects. This means that one's body requires larger amounts of a drug to have the same effects that lower doses used to create.
Most depressants rapidly produce an extreme tolerance.
Physical dependence to a drug occurs when an individual's tissue cells get used to a certain level of intoxication. This means the body physically changes, needing the drug in regular doses to maintain its chemical balance. Psychological dependence to a drug occurs when a person needs regular doses of the drug to maintain mental and emotional stability. In either case, a drug dependent person will experience some form of withdrawal symptoms when drug use is discontinued. Opioids are synthetically produced opiates such as methadone, propoxyphene and fentanyl.