Also heard: "I only light up when I'm drinking."

Why you should 'fess up: If you smoke, you're at a higher risk for lung, cervical and breast cancers, and your doc needs to start early or more frequent screenings for these diseases. Smoking can also interfere with meds -- it makes patients with early rheumatoid arthritis less likely to respond to the two most commonly prescribed medications on the market. And even occasional smoking raises your risk of blood clots and stroke, especially if you're on hormone-based birth control such as the Pill, patch or ring.

Smoking also affects post-op healing because it decreases blood flow. Why is this a problem? Here's an example: "Skin cancer surgeries require using a skin flap or graft to reconstruct the defect once the skin cancer has been removed. Decreased blood flow can significantly increase the chance of the flap or graft dying and falling off," says Dr. Soheil Simzar, a Santa Monica dermatologist at Ava MD. Decreased blood flow also means higher infection rates at the surgery site and higher risk for complications from general anesthesia, says Melissa Doft, a New York City plastic surgeon. She recommends patients stop smoking four weeks before and four weeks after a surgery.

Also heard: "I don't do drugs."

Why you should 'fess up: Your doctor isn't your mom or dad. Lying about your recreational drug use puts you in an extremely dangerous situation if you're undergoing surgery and could signal a deeper, underlying drug problem that could potentially spiral if your doctor is prescribing addictive medication.

Recreational drugs come in two categories these days: legal (think prescription painkillers) and illegal (think cocaine). If it's legal, you may "forget" to mention that you were prescribed Oxycontin for an injury and maybe took the drug for a bit longer than medically required. "If I know a patient has had substance abuse issues in the past, I'm more likely to avoid medications with addictive potential to keep the problem from spiraling," says Alabama-based Dr. Saundra Dalton-Smith.

If you dabble in the illegal variety, your doctor isn't going to rat on you and have you arrested. On the flip side, if your doctor doesn't know you smoke weed or occasionally do coke, surgeries and anesthesia become dramatically more dangerous and risky. "Patients who are not actively on a drug like cocaine may still have organs effected by chronic drug use. This could mean needing different drugs during an operation to control blood pressure and cardiac arrhythmias, but if the anesthesiologist and I don't know that, it's extremely problematic," says Doft.

Why you should fess up: As harmless as it seems, that herbal supplement designed to boost energy, improve your memory or make your skin glow may contain ingredients like caffeine or green tea extract, which can conflict with other medications you're taking, says Agarwal. Example? St. John's Wort. This herbal supplement is used to treat depression, anxiety and tiredness, but it also makes other prescriptions like birth control and anti-rejection drugs less effective. If you're leaving this out, you could be setting yourself up for unwanted pregnancies and botched organ donations.

Certain supplements and over the counter meds also interfere with surgical procedures. "Aspirin, ibuprofen and certain vitamins, like fish oil, will make you bruise more easily. Use of Accutane in the last year can even affect how you heal and whether we should wait on performing a procedure or not," warns Doft.

Also heard: "Everything's fine. Nothing new."

Why you should 'fess up: You made this appointment for a reason, and expecting your doctor to be a mind-reader wastes time and increases your chances of your problem getting worse (or, at least, not better). Doctors have heard it all -- if you're there because you're depressed, exhausted, have no sex drive, can't sleep, etc. -- welcome to the club. According to Smith, the most common issues people are shy about discussing are loss of sexual libido, fatigue, anger, depression, anxiety, and fears they may have something like cancer or heart disease.

"People will say they don't have a specific concern and just want a check-up, but 70 percent of the time, they actually have something that's been troubling them and they are praying I will be able to magically tell them the answer to questions they haven't been bold enough to ask me," says Smith. "If you can't talk about these things with your doctor, who are you supposed to talk about them with?" If it's that much of a struggle to open up to your doctor, it's time to find a new one you feel 100 percent comfortable with.

Also heard: "No, I'm not taking anything," and "Just birth control ... "

Why you should 'fess up: Your doctor may inadvertently overprescribe or take you off a medication because they think it's not working, when in reality, you're just not taking it correctly. Instead of digging yourself a deeper, potentially life-threatening hole, you need to admit your poor pill-taking habits.

"When cholesterol numbers are still high and the blood pressure is not improving on therapy, my first question is often "Are you taking the medication as prescribed?" Many people are ashamed to tell you they forget and miss pills regularly, so rather than fessing up they just lie," says Smith. Ironically, this attempt to save face often leads to patients being unnecessarily over-medicated. "If a low dose of the medication does not work, most doctors will increase to a higher dose, which may not be necessary if someone is not taking the medication regularly."

Omitting other medications you are taking regularly because they may seem unrelated to the specialty can be just as dangerous. You're probably OK letting your OBGYN know that you're birth control, but what about everything else? You may be embarrassed to admit that you're anti-depressants, but Dr. Hal Danzer, co-founder of Southern California Reproductive Center, says these are especially important to bring up because when taken during pregnancy, selective serontonin reuptake inhibitors (SSRI) such as Prozac, Zoloft, Celexa and Lexapro can increase the likelihood that a baby is born with septal heart defects.