Juan finds that he must take more and more amphetamines to get the same rush as he used to feel. This is an example of: a) Physical dependence b) Addiction c) Psychological dependence d) Drug tolerance

d) Drug tolerance

The phenomenon described in the question is known as drug tolerance. This occurs when the body becomes accustomed to a drug, requiring higher doses to achieve the same effect. This is not necessarily indicative of addiction, physical dependence, or psychological dependence, although these can sometimes co-occur. The key point here is the increasing need for larger doses to achieve the same effect, which is the definition of drug tolerance.