system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a medicine, like one to help quit smoking, develop into side effects like suicidal thoughts?
gpt
Because it changes the chemical composition of your body, specifically your brain chemistry.

Think of it like this: Medicine A works by causing your brain to produce lots of chemical A.  Chemical A being present in your system helps to curb your desire to smoke, which is great.  But chemical A does lots of things besides that... maybe it regulates your heart rate in some way, maybe it affects your mood, maybe it affects your memory.  One of those many side effects could increase suicidal thoughts.