A striking number of CNS-active drug classes, from mainstream antidepressants to antipsychotics to migraine treatments to some recreational hallucinogens, all act on the same broad neurotransmitter system: serotonin. That's less strange than it sounds once you know that serotonin doesn't have just one receptor — it has more than a dozen distinct receptor subtypes distributed across different brain regions and functions, and a drug's actual clinical effect depends far more on which of those specific subtypes it binds, and whether it activates or blocks that subtype, than on the fact that it's "a serotonin drug" in general.
One neurotransmitter, many different locks it can fit
Serotonin receptor subtypes are distinguished by researchers using a standard naming system, grouping them into several families, each associated with somewhat different downstream effects depending on where in the brain and body they're expressed. A drug that increases available serotonin generally, without much specificity for which receptor subtype ultimately gets activated, will produce a broad mix of effects across all the systems that subtype family influences, which is part of why many serotonin-affecting medications carry a wide range of possible side effects alongside their intended one — the drug isn't targeting a single narrow effect, it's shifting activity across an entire signalling system with many different jobs.
Selectivity is what turns a blunt tool into a precise one
Much of the ongoing development in CNS pharmacology has focused on finding molecules that act more selectively on one specific serotonin receptor subtype rather than the system broadly, precisely because greater selectivity tends to produce a more predictable, more targeted clinical effect with fewer unrelated side effects. A drug developed to selectively activate one particular receptor subtype associated with migraine relief, for instance, can relieve migraine symptoms without meaningfully affecting mood-related receptor subtypes elsewhere in the same broad serotonin system, even though both drugs are, in the loosest sense, "serotonin drugs." The clinical differences between drug classes that all touch the same neurotransmitter system come down almost entirely to this kind of receptor-level selectivity, not to the neurotransmitter itself.
What we're still unsure about
The existence of multiple distinct serotonin receptor subtypes, and the principle that receptor selectivity drives a drug's specific clinical profile, are well established in pharmacology. What remains genuinely difficult, and an active area of drug development, is designing molecules selective enough in practice to reliably hit only the intended receptor subtype without meaningfully affecting related ones, since many receptor subtypes are structurally similar and a drug's real-world selectivity in a living brain doesn't always match its selectivity in isolated laboratory testing — which is a substantial part of why CNS drug development remains slow and side-effect profiles are often only fully understood after extensive clinical testing.
This sits inside CNS Pharmacology (Analgesics, Antidepressants, Antipsychotics), one of seven topics in Pharmacology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.