You're in the exam room in your doctor's office, and it's the moment of truth. She's just asked you a personal question, one you'd rather not answer. Only your best friend knows the facts on this one. In fact, just thinking about it makes you squirm. Do you really need to be completely honest with your doctor? How much difference does it make if you say you enjoy "a couple drinks a week" when the real number is more like a dozen? And does it really matter if you claim to exercise an hour a day when most of that involves walking from the couch to the fridge?

We're all taught to be truthful, but that doesn't stop people from fibbing to their doctors. Ask any health professional-patients frequently play fast and loose with the truth. Common areas for fibbing include drinking, drugs, smoking, sex, and diet and exercise. In fact, many doctors who are wise to patients' untruths will automatically make allowances for them. An admission of two drinks per weekend becomes four drinks per weekend in the doctor's mind. And some of what patients lie about is patently obvious. If you've gained 10 pounds in the past year but claim to have stepped up your exercise and cut your calories, the doctor has probably figured out that you're not owning up to the truth.

But can these lies come back to bite you later? Yes, doctors and other health professionals say. Particularly dangerous are situations in which patients don't admit to taking certain drugs or herbs-legal or illegal-which could interact with other prescriptions. "If you come to the hospital and we give you a medication, it may interfere with [whatever else you're taking]," asserts Athena Lee, a physician assistant in New Jersey. "You've got to know when to tell the truth." Some drug combinations, she says, can be lethal. Never a good mix? Viagra, taken by men to enhance sexual potency, plus nitrates, which are commonly given for chest pain or congestive heart failure...

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