thank you for the explanation! i’d like to add a fun fact (it’s actually a sad fact) about dopamine that wasn’t mentioned in this chapter: if seungmin ends up being the trigger of dopamine release, then jaeyoung will only feel pleasure BEFORE spending time with seungmin, because the dopamine rush is tied to anticipation and expectation rather than the experience itself. which means seungmin will never witness jaeyoung at his happiest. sad, isn’t it?

for people that are confused about the drug and Dr. Shin's explanation in ch24, i hope this helps a bit ^^
neurotransmitters are chemicals that allows our brain neurons to communicate with each other. serotonin is an NT that stabilizes our well-being, regulating our mood, sleep, etc. it's what allows us to feel satisfied.
how the Smile drug works is that it overloads the person temporarily with serotonin, leading to a period of intense euphoria and heightened senses, before destroying the serotonin receptors that would normally receive serotonin -- so there's a breakdown in communication. if your body can't receive serotonin chemicals, it leads to depression, sleep disorders, anxiety, memory loss, and more. it sounds like the Smile drug is an extreme version of molly/ecstasy, if you're familiar with that!
we have another NT called dopamine, which is known as the pleasure NT: we receive dopamine when we're doing things that make us feel good, it's like a reward, but it's only a temporary rush (compared to the long-term calmness we get with serotonin).
what Dr. Shin is suggesting is basically rerouting the serotonin away from the damaged serotonin receptors, and towards the undamaged dopamine receptors to be received alongside dopamine. IIRC, this is how some SSRIs/antidepressants work! however, it seems that normal SSRIs haven't worked on Jaeyoung. I'm not exactly sure of Seungmin's role in this, but maybe (with the power of BL logic) Seungmin's giving Jaeyoung experiences that would lead to dopamine and serotonin release while grounding him so he won't receive too much of it (because that would lead to aftereffects like with cocaine). basically, like a human SSRI?
if anyone more versed in neuroscience wants to pitch in please do so !! this is a very simplistic explanation based on my limited neuropsychology knowledge lol