Because our bodies are delicate systems of networks, and inputs in one area can have complex/unpredictable outputs elsewhere, it seems.

Typically, if something "works", there often appear to be side effects. A free lunch is rare.

https://www.uclahealth.org/news/article/understanding-medica...

> Typically, if something "works", there often appear to be side effects

Unless it’s literally a deficit. There isn’t a downside to treating vitamin C deficiency with vitamin C.

The fact that GLP-1 seems to have roles not just in satiety but that agonists seem to reduce other types of impulsiveness (e.g. gambling, shopping) is interesting. That's not something you'd predict as a consequence, and perhaps is downstream of some gut-brain connection.

Of course we already manipulate brain chemistry in other more direct ways with antidepressants so perhaps any unwanted second-order effects could be minor in comparison to the profile of existing antidepressants .