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.

Also heard: "Only my ears are pierced."

Why you should fess up: "If a patient is undergoing surgery and electrocautery (passing currents through a metal wire to generate heat) is used to control bleeding, the current can run through a metal piercing and fry that skin ... oops," says Dr. Barbara Bergin, an orthopedic surgeon in Austin, Texas. Earrings are noticeable enough, but if someone is having knee or shoulder surgery, there's no need for nurses or doctors to examine certain, uh, obscure body parts. "Please just take out your piercings and leave them at home, or remember to say 'yes,' when the nurse asks you if you have any body piercings," says Bergin.

Also heard: "I wear sunscreen every day."

Why you should fess up: If you aren't honest about regularly baking in the sun or roasting on a tanning bed, it makes it harder for your doctor to detect skin cancer early on. Simzar says most of the patients he sees lie about about sun exposure they've accumulated or tend to leave out noticeable chunks in their tanning bed history. According to Simzar, a physician informed with your true tanning habits will be better equipped to find suspicious lesions and samples to send over to the lab for analysis -- something that could mean the difference between life and death.

Other lies that float around a derm's office have to do with conveniently overestimating hand-washing frequency and sunscreen use, underestimating picking at the face and leaving out past products and procedures, all of which affect the outcome of prescribed medications and how well or soon your skin will start to heal according to Dr. Andrea Paul, the chief medical officer of BoardVitals in New York. For example, high levels of retinol can make skin more sensitive to the sun and more susceptible to tearing. "I once saw a patient who was using retinol on her face and had a waxing done, but failed to disclose to them and to me that she was using a retinol product," says Paul. The end result wasn't pretty. Retinols chemically slough off the outer layer of dead skin cells, meaning the wax wasn't just ripping out hairs, but it took living skin cells along with it.