What causes dark circles beneath the eyes?

QUESTION: One thing that's really bugging me (and, of course, it's probably age-related) is this dark circle I've developed under my right eye. I have allergies but even when my allergies seem to be in check, it's still there. And why would I have a dark circle under just one eye?

_ J.R.

ANSWER: It is true that people with nasal allergies may have dark areas under the eyes. These are called allergic shiners and appear when blood pools in veins around the eyes because its flow is limited by congestion. The same thing can happen from other causes, such as trauma (shiners) or unilateral nasal blockage (septal deviation, enlarged adenoids or polyps). Subcutaneous bleeding at the forehead or eyebrow level from trauma or other causes may drain to a level at or below the eyes. In your ...