The speaker describes taking a pill as if it were a replacement for a wife, and the poem seems to be about grief, or a marriage that has ended, and the numbness that comes with trying to medicate it away. The wife is "made of loss and ash," a pretty image that is also bleak: she is already something burned and gone, more memory than person. Judging by the title, there is a bitter joke at work too. "A taste of her own medicine" suggests payback, but the poem turns it inward, since the speaker is the one swallowing.
The second stanza calls the whole thing "a fable," as if the speaker is telling himself a neat story to get her "out of the way." The last lines are the saddest. The "door without a house" is an opening that leads nowhere, and it disappears "like a pill," quickly and without a trace. The mood is quiet, dry and hollow, with dark humor covering a real ache.