Out of curiosity, when some medication causes weight gain, how does it work? Does it increase appetite? Or does it slow metabolic rate?
It increases water retention (obviously not permanent or unbounded), increases appetite, and redistributes fat (giving the appearance of weight gain).
A potential side effect of immunotherapy is it can cause the immune system to go haywire and start attacking non-cancer cells.
0. https://www.medicare.gov/coverage/obesity-behavioral-therapy