A person hides their hurt behind smiling and laughing, and the poem is about what happens when that disguise stops working. The speaker describes a girl who put on cheerfulness the way someone takes a pill: to dull the pain, not to cure it. Nothing in the poem suggests the pain was ever dealt with. It was only muffled for a while.
The comparison to medicine is the heart of the poem. Medicine brings relief, but it fades, and the ache returns once the dose is gone. By calling her laughter "medicine," the speaker suggests it was a kind of treatment she relied on, and also that it was temporary from the start. The last line, "It wore off eventually," is flat and quiet, with no dramatic outburst. That plainness makes it feel sadder, as if collapse is simply the expected ending.
The mood is gentle but heavy, a sympathetic look at someone whose happiness was only a cover, told by a speaker who seems to understand how tiring that act can be.