molly is mdma! mdma is the scientific name for molly/ecstasy
There's no real drug that permanently destroys all your serotonin receptors or production, as far as I know, and it's definitely not MDMA or molly. The medical talk is a bit opaque, but they're using a fictional street/designer drug to make this story. It's not impossible, just as fentanyl is a super potent opiod, some super potent serotonergic agonist could maybe cause excitotoxicity (overactivity that leads to cell death) and permanently damage serotonin pathways, but molly won't. Also, if your entire serotonin system is fried, you have more to worry about than just mood, since it's involved in modulating several other processes like memory and digestion.
Ok I fact checked quickly, and there are toxins deliberately used in research settings to destroy parts of the serotonin system (5,6- and 5,7-Dihydroxytryptamine), so "no drug exists" was an overreach, but to keep it simple: no, it's not MDMA/molly, but it's some fictional drug in that neighborhood.
Is the drug Mdma? or molly