system
Given the following post, respond as if you're commenting on Reddit:
human
How is caffeine physically addictive and why does a person experience withdrawal?

What causes caffeine to be physically addictive, so that an addict experiences withdrawal when they do not get a dosage?
gpt
Caffeine acts as a competitive inhibitor for the neurotransmitter Adenosine. It does this by binding to adenosine receptors without activating them.  During continuous use, the body works to counteract this effect by increasing the number of adenosine receptors in the central nervous system.  Because of this adaptation, the body is then more sensitive to adenosine in the absence of caffeine.  The body's heightened reaction to adenosine involves the unpleasant symptoms typically referred to as withdrawal.