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Tolerance means a person's body has adapted to a substance to the point that they need more and more of it in order to have the same reaction. With many drugs, the first use has a greater physical impact than any time afterward. That's why there is a tendency to increase their intake over time, to try and recapture that initial high. Tolerance is a common symptom of addiction but Compton says that just because someone has developed a tolerance, doesn't mean they're addicted.