system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some drugs, like opiates, develop a tolerance in the user while other drugs, like Prozac, do not?

Just wondering why with some drugs the brain adjusts to the new balance of brain chemicals, whereas with SSRIs and such, it doesn't.
gpt
This is a tough question to ELI5, but drug tolerance is controlled by a complex series of counter-regulatory systems, and some systems are simply more responsive than others.  

Opiates are an interesting example because they act on multiple receptors, and only some of those receptors develop tolerance.  The opioid receptors that reduce pain sensation and cause euphoria become much more tolerant over time, while the receptors in the GI tract and respiratory center of the brain do not develop tolerance.  Because of this, people need increasing doses to relieve pain and/or get high, but they do not become resistant to the respiratory and GI side effects.