It starts out innocently enough. "I don't smoke ... " That's technically true -- you just forgot to tack on " ... Monday through Thursday," right? If completing that checklist in your doctor's office means lowballing your vices or glossing over certain unsavory details in your past, those little white lies could lead to major medical issues (yes, we're talking about the c-word).

Are you guilty of these dangerous lies? Now is the time to come clean.

Also heard: "I only have one glass of wine a day ... "

Why you should 'fess up: Your MD isn't judging your drinking habits. He's trying to gauge the best medications to prescribe. Moderate to heavy drinking can dangerously interfere with certain Rxs, like causing sudden changes in blood pressure with patients on heart medications, internal bleeding with patients on blood-thinning medications and impaired breathing for those on sleep medications. If you're lucky, your doctor is wise to the fibbing. "I'll usually double whatever the patient claims to drink to get the actual number," says Dr. Bola Oyeyipo, a family physician in San Antonio, TX. "Most women underestimate how much they drink, partly because we tend to have big pours for wine and cocktails and because they don't want to seem like they are drinking heavily."

Your doctor will also give you better boozing advice. "Too much alcohol at once bumps up your risk for breast cancer, so I want women to understand that if possible, one drink every night is actually better than seven drinks all at once," says Dr. Shilpi Agarwal, a family medicine specialist in Los Angeles, Calif.

Also heard: "We always use a condom."

Why you should 'fess up: Keeping your sexual history under lock and key prevents your doctor from keeping a watchful eye for STDs that can hurt you down the line ... and from prescribing you meds that could potentially harm a fetus. Don't be embarrassed -- doctors have literally seen and heard it all. "I have had teenagers tell me they've never had sex only to get back a positive pregnancy test or a positive STD test," says Dr. Bola Oyeyipo, a family physician in San Antonio, TX.

OK so, that STD you got -- and treated -- way back when, or that night of unprotected sex with some dude whose name you never caught: You can pretend like it never happened, right? Wrong. Even if your initial tests came back clean, it can take up to six months for HIV to show up, says Agarwal. If you're at risk, your doctor will insist on a follow-up test. Plus, gonorrhea or chlamydia increase your risk for ectopic pregnancies -- your doctor needs to know this history to keep a close eye on you if you do get pregnant.

Oh, and this goes double for spilling the beans about your partners' sexual history. Don't hold back if your partner cheated, or you slept with someone who sleeps around (which is safe to assume if it's a one-night-stand).

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.