Q.    What is tolerance.

DR. CLARK:             With frequent use, or chronic use, you change the brain's ability to function normally. Some of those ways are not as clear, as evident, as other ways. 
 
DR. SHAFFER:                          All drugs that affect the way you feel are affecting your brain, they're influencing how the brain works, they influence the chemistry of the brain, and they can influence the way the brain cells communicate with each other. 
 
DR. CLARK:             One of the reasons that people do drugs is because it affects areas of the brain that causes them to feel high or feel pleasure. We call those the pleasure centers. So they stimulate those pleasure centers using these drugs and become addicted by having the repeated stimulation of the pleasure centers and discovering that the pleasure centers don't respond as well with the chronic use. So each time I use the drug, the pleasure center responds less and less.