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You lie to your doctor

Many of us don't tell our doc everything—say, we smoke cigarettes or drink more than we should. Or we may take that prescription with no intention of ever filling it. Harvard Medical School researchers found that more than one in five first-time prescriptions never got filled (this was especially true for chronic conditions such as high cholesterol, high blood pressure, and diabetes).

But you're better off being straight with your doctor, who can help you find ways to kick the habit or find a treatment you'll actually take (or be able to afford).

If you don't feel you can be fully honest with your doctor, says Dr. Nundy, you should look for a new one.

Next: You think you know more than your doc