Antidepressants

Chronic-dosing mood agents: the reuptake inhibitors (SSRIs, SNRIs, tricyclics), the MAO inhibitors, and an atypical group defined by being neither. They are on the site for interaction safety rather than for their own sake, and the dosing schedule is the defining fact — somebody on one of these is at steady state, so the interaction is present on a day they took nothing else and cannot be timed around.

Serotonin syndrome lives here: serotonergic agents combined with each other, with a releaser like MDMA, with tramadol, or with an MAOI can produce it, and the picture is agitation, tremor, rigidity and rising temperature rather than anything gentle. Two other class-level facts. Several members inhibit the CYP enzymes that clear the rest of this tree, so an unchanged dose of something else is not an unchanged exposure. And stopping abruptly has a discontinuation syndrome of its own.