A friend told me about a recent meeting with a new doctor, where she dutifully filled out all the required paperwork. “Hmm,” the doctor said, scanning her forms. And then she stopped and gasped.

The issue was the number next to “Drinks containing alcohol per week.” My friend did the unthinkable: she told the truth. “I figured two glasses of wine a night, so that makes 14.”

The doctor was horrified and suggested Alcoholics Anonymous. My friend was horrified and found a new doctor.

For all the talk about transparency and authenticity, for all the yearnings for wellness and longevity, there seems to be a blind spot. We lie to the very people who are doing their best to safeguard our health.

Not all of us; that would be false. But according to research by Brigham Young University and the University of Utah, a full 60 to 90 percent of patients lie on their medical forms. It’s a problem that compromises health, causes misdiagnoses, and worse.

They lie about smoking, exercising, drinking, and sexual partners. They lie because they’re embarrassed, and because they’re ashamed. They lie about Xanax. They lie for Xanax. They lie because they want the doctor’s approval. They make wishful statements such as “I try to work out six days a week” or “My goal is to eat less sugar.”

That’s when Amy B. Bleyer, an internist in New York City, replies, “O.K. And how often do you meet your goal?”

“There are the patients who specifically tell a tale that they think the doctor wants to hear,” says Bleyer. It’s not called a medical exam for nothing. Some see it as a test to ace. “You want to act like the perfect patient. You want to suggest that you don’t have any problems,” says Steven Teitelbaum, a board-certified plastic surgeon in Santa Monica.

Doctors are so prepared for deceit about alcohol consumption that they often adjust the numbers on the forms. “The mantra throughout my training was, if a patient admits to 6 to 10 servings of alcohol a week, you double it,” says Dr. Bleyer. “That was the conventional wisdom. Now, every time I type in a number, I think, Is this an accurate number, or should I be doubling it?

(By the way, the upper limit for alcohol is 7 drinks a week for women and 14 for men, but many doctors have rounded that down to a solid zero.)

Pamela Meyer, the author of Liespotting: Proven Techniques to Detect Deception, says, “Most lies to doctors are likely lies of omission. Lying is, at its core, an attempt to bridge the gap between how we wish we were with what we really are like. We lie to avoid conflict, embarrassment, to protect others, protect our privacy, and to shift our image and presentation to others.”

We lie to the very people who are doing their best to safeguard our health.

Some of these lies can put patients in real danger. Dr. Teitelbaum is unyielding about nicotine, and he has a specific intake form focused on it. “In procedures where we cut under the skin and pull it under tension—and that’s the facelift, the breast lift, and the tummy tuck or breast reduction—the nicotine constricts the very small blood vessels, and you’re dependent upon those small vessels to keep that skin alive and make the edge of the incision heal.” Nicotine use can compromise the healing process, he says.

And yet, patients tell him they don’t smoke even when their clothes are practically smoldering. “If they smoke, I give them a consent to sign that says they understand that smoking can have a severe impact on their healing.” He does this in part to “invoke terror” and, ideally, get them to quit. On the morning of surgery, “we do a urine test, and it will tell us if you have any by-product of nicotine in your urine,” he says. If the test is positive, he cancels the surgery without a refund. “And that lets people know that I’m serious.”

People lie about taking Ozempic or other semaglutides, perhaps not knowing their danger for anyone under anesthesia. According to Dr. Teitelbaum, “They delay gastric emptying,” meaning the stomach may not be empty during surgery. “That’s not something to lie about,” says Dr. Teitelbaum. “People die from that, even young and healthy people.”

Some patients “forget” to tell their surgeon that they’ve had previous plastic surgeries, which seems especially dizzy. “It happened just yesterday,” says Jason Diamond, M.D., a plastic surgeon with offices in Beverly Hills, New York, and Dubai. “A patient came in for a facelift consult. I said, ‘What did you have done?’ And she said, ‘A little lipo around my neck.’” When he pointed out the scars by each ear—evidence of a full facelift—the patient tried to argue with him. “You can’t hide a facelift scar from me,” he says. Good thing, because a previous surgery and its scar tissue could change the outcome of subsequent procedures.

Sometimes, patients lie to Dr. Diamond about having a thread lift, which involves inserting barbed strings under the skin and lifting them to give a so-called mini-facelift. “So many people have had them and they won’t tell me. I don’t know if they’re ashamed because they did something that didn’t work. And they never work.” Post-operatively, Dr. Diamond will inform the patient that he found evidence of the thread lift. “And they’ll say, ‘I didn’t remember.’”

All of this would be comical if it weren’t so troubling. Studies indicate that incorrect medications can add billions of dollars to health-care costs, not to mention the physical risks. That’s why doctors try so diligently to coax out the truth. Dr. Bleyer does it by reminding patients, “There are no judgments here. The things that I’m recommending are only for your benefit. You can do them. You can not do them.” That usually elicits a confession. In the B.Y.U. and University of Utah study, women tended to tell the truth when the doctor questioned them further, while men often doubled down on their lies. “I can’t really help patients prevent future disease if I don’t know an accurate story,” says Dr. Bleyer.

Meyer leads a firm, Calibrate, that trains professionals—lawyers, insurance providers, human-resource executives, and financial advisers—in how to detect deception. Her TED Talk, “How to Spot a Liar,” has more than 33 million views. “The key is to normalize what the patient might be embarrassed to tell you. We do this all the time when training investigators and interviewers. We teach them to leave enough time for the subject to come forth.” In other words, the truth will out. She discusses micro-expressions, body language, and false denials. And while that sounds easy for Natasha Lyonne on Poker Face, some doctors aren’t so sure.

“I’m not a C.I.A. interrogator,” says Dr. Teitelbaum. “I have no ability to look at your eyes and say, Oh, she was looking up into the left, and she twitched her nose.” He does, though, work to establish trust by spending time listening to his patients.

And sometimes he downplays the facts. “You need to be honest, but if you scare people too much they won’t do surgery.... With almost the same words, you can make a procedure sound very risky or not very risky.” At least he knows where we’re coming from.

Linda Wells is the Editor at Air Mail Look