Are you honest with your doctor? Come on, completely honest? Chances are you've held back some details you'd rather not share. A recent survey found that 28 percent of people admit they sometimes lie to their healthcare provider or omit facts about their health.

Many of us think there’s nothing wrong with telling a little fib to the person in the white coat — but these untruths or omissions can have big health consequences. “The goal of healthcare is to keep you healthy,” says Reid Blackwelder, MD, chair of the American Academy of Family Physicians’ board of directors. “As a doctor, the more I know about your habits, the better able I am to keep you healthy.”

Here are seven lies you should never tell, and why what your doctor doesn’t know can hurt you.

Related: 10 Health Symptoms Women Should Never Ignore

1. “I never smoke.”

Even if you smoke only “socially,” clear the air with your doctor. “Even occasional smoking creates health risks,” says Blackwelder. You probably know most of them: lung, breast and oral cancer; heart disease; chronic obstructive pulmonary disease (COPD); and emphysema.

If your doctor knows how often and how much you smoke, he’ll have a better sense of how at-risk you are. That information also can help him monitor you for problems caused by smoking. For example, if someone who smokes is wheezing but says he never lights up, “it makes it hard for me to manage the problem properly,” says Blackwelder. “If I don’t have the information I need, I may order diagnostic tests, which carry costs and potential risks, to find out what’s causing the wheezing.”

2. “Drink? Who, me?”

Do you have only the occasional glass of wine? Or is a regular nightcap or two more your style? Maybe you binge drink on weekends with the girls. ‘Fess up.

The reason: Alcohol use is a risk factor for heart disease, high blood pressure, memory problems, depression and various cancers, including breast, esophageal, liver and colon. If you’re pregnant, alcohol can harm your developing baby. 

Too much booze can skew the results of liver tests as well. “If I’m assessing your overall risk for heart disease, cancer or other health problems, it’s important I know how much you drink,” says Kittu Garg, MD, an internist at Cleveland Clinic. When you’re upfront about your drinking habits, your doctor can better counsel you about health risks and determine if you need professional help.

3. “Yup, I floss twice a day!”

In fact, you can’t remember the last time you bought floss, much less used it. You’re not the only person who fibs about flossing. According to a survey from the American Academy of Periodontology, 27 percent of adults admit they have lied about how often they floss.

Why should you tell the truth? For one, if you don’t floss because it’s too difficult, your dentist can recommend tools to make it easier. Or maybe bleeding is the reason. Tell your dentist and you’ll likely be reassured that’s normal. “It means you may have a wound under the gum and you’re cleaning it out,” says Joan Otomo-Corgel, DDS, president of the American Academy of Periodontology.

Another reason to be honest: If you fib about flossing and your dentist spots biofilm — or plaque — on your teeth, she may suspect you have dry mouth, which ups your risk for cavities and may prompt her to recommend you see a periodontist or boost your fluoride, when all you really need is to take a trip to the drugstore for dental floss.

Related: 11 Health Symptoms Men Should Never Ignore

4. “The only OTC pill I pop is the occasional ibuprofen.”

If you take any kind of medication or supplement regularly or even just frequently, your doctor needs to know — especially before he pulls out his prescription pad to write an Rx for something else.

Supplements and OTC medicines can interact with prescription drugs. “If I know what you’re taking, I can adjust the dosage of whatever I prescribe,” says Garg. Honesty can also prevent problems. Doctors commonly prescribe an ACE inhibitor to treat high blood pressure, for instance. But combining the drug with an OTC medicine like ibuprofen raises the risk of kidney failure.

It’s also important for your doc to know if you regularly use a non-steroidal anti-inflammatory medicine, such as ibuprofen or naproxen, since NSAIDs raise your risk of suffering a heart attack or stroke. 

Being briefed about your OTC use can help your provider figure out what might be causing certain symptoms, as well. For example, if you have kidney disease and take NSAIDs regularly “you could have a sudden worsening of kidney function,” says Garg. A first step would be to stop the NSAID and see how you do, but, explains Garg, “if I don’t know about it, I would start investigating other causes.”

5. "I'm taking my medicine." 

You also need to come clean if you aren’t taking medication you’ve been prescribed. Say you didn’t finish your course of antibiotics for a sinus infection and now it’s worse. Or you stopped taking your blood pressure medication because you hated the side effects. What may happen is your doctor will prescribe a different and possibly more powerful antibiotic to treat your infection, putting you at risk for antibiotic resistance; to lower your blood pressure, he may give you a more potent and pricier drug with worse side effects. “I want to know the issue so I can work with you on it,” says Blackwelder.

Related: Prescription Painkillers: Know the Risks

6. “Ummm… my sex life is just fine. Perfectly normal. Can we talk about my ingrown toenail now?”

This is a toughie. Who wants to share intimate secrets? But you should. “Discussing sexual habits isn’t a natural thing to do,” acknowledges Blackwelder. “But I need to be aware of things that are important to you.”

For example, if you’ve reached menopause and want to be sexually active but intercourse hurts, the doctor may be able to help, if she knows about the discomfort. 

7. “I work out every day.”

If your doctor has prescribed exercise for early-stage diabetes or obesity and you tell him you’re exercising full tilt, yet your blood sugar or weight hasn't budged, she may prescribe medication or a more aggressive therapy. “More strenuous types of treatment could be avoided if I know exactly how much you’re exercising,” says Garg. If the real answer is "not much," admitting it may help you get the motivation you need to turn your little white lie into a bright and shiny truth. 

Catherine Winters is a freelance writer who specializes in health.