Tolerance means that the intensity of the drug's effects has decreased, and so larger doses of the drug are required to produce the same effect. While a tolerance to the desired effects builds up, little tolerance develops to the lethal toxicity of these drugs, so a tolerant individual is just as susceptible to death as a non-tolerant person. In a situation in which the user is constantly increasing the dose to feel the desired effects, it is not hard to imagine how a tragic drug overdose can occur. For many drugs, tolerance build-up quickly leads to physical dependence. In this case, a person's body has become so adapted to the toxic drug that it comes to depend on it as a part of normal functioning.
Any drug can bring physical dependence, but it is most common with sedatives and narcotics.
Another kind of dependence that builds-up is psychological dependence. People with drug cravings will often go to extreme measures to get the drug, disrupting their personal lives, their income flow, and sometimes society at large. The degree to which a person becomes independent depends on the drug, the person, and the frequency and amount of dosage. Once a person becomes dependent, he is generally considered an addict. Recent studies have shown that the difference between physical and psychological dependence is not that significant.