Mendon began by taking one amphetamine a day to control his appetite. after a month or so, the one pill did not work as well but two pills did. this is an example of:

The answer to this question is tolerance
Tolerance refers to the ability to accept something different even though we're not fully agree about it.
Example:
Kate has is a liberal and Dave is a conservative. But rather than fighting each other like the news stations trying to do, they decide to acknowledge the positive values of each belief and befriend one another.