Edit: Thank you very much for the gold! I hope the comments made to this post were as helpful to others as they were to me.
In: Biology
Another good example is the overexpression and/or increased activity of drug efflux transporters. These transporters are present on the cell membrane and can export drugs out of the cell, thereby reducing the overall intracellular concentration and reducing the efficacy of the drug.
Alternatively, there may also be increased expression/activity of enzymes such as the CYP450 family which metabolise drugs into a non-active form (if the drug isn’t a pro-drug), similarly reducing the concentration of active drug in the system. The activity and expression of these enzymes can be pre-determined by our very own genetic make-up but also modulated from the things we consume (food, drink, other medication), things we are exposed to (chemicals in cigarette smoke) and even our current state (age, pregnancy).
As for the specificity of drugs which a tolerance is built up towards. These drug efflux transporters and enzymes typically interact with a subset of drugs and it is these drugs which become less effective over time.
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