Answer: Tolerance
Explanation:
The more we take a certain substance, the more the body becomes adept at dealing with it which would mean that more quantities would have to be taken each time in order to try to get to the original effect the substance caused.
This is called tolerance. The body is getting better at tolerating the substance. This is the reason alcoholics drink more to be intoxicated than non-alcoholics. It also explains what happened to Avery here.