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A number of years ago, a doctor colleague of Marvin M. Lipman, Consumer Reports’ chief medical adviser, saw a nursing student who wanted help with weight loss. After several months without shedding much weight, she began to drop off pounds in dramatic fashion. Suspecting diabetes — which can initially cause rapid weight loss — or worse, the doctor did a workup, which disclosed an overactive thyroid. He then referred the 19-year-old woman to Lipman.

Before discussing treatment, he decided to recheck her lab tests and added a few more. Although the young woman initially denied taking medication, the test results clearly indicated that she was using the drug Cytomel to lose weight. It speeds up metabolism and can be harmful to the heart, muscles and bones. When he confronted her, she broke down in tears and confessed.

Why patients lie

Patients lie or withhold information from their doctors for a variety of reasons. Some do it because they’re embarrassed about discussing sensitive topics such as sexuality; others because of shame and guilt about unhealthful habits such as smoking, or about abusing drugs or alcohol. In a study from Spain that analyzed the breath of people with lung disease for carbon monoxide — a gas that is found only in active smokers — one out of three who tested positive denied smoking.

Another reason patients lie is the desire to please. One case in point involved a woman with Type 2 diabetes who impressed Lipman with the near-normal fasting blood sugar levels she reported. That is to say, he was impressed until he downloaded her home blood glucose meter and found out that her diabetes control was not as good as she had claimed. He was then able to adjust her medications to help her achieve better control and perhaps avoid such eventual complications as damage to her eyes and nerves.

Falsehoods are most prevalent — and potentially most harmful — with respect to following instructions when taking medication. Research has found that more than half of patients tell their doctor they’re taking medications as directed when they’re not. In a study that used a device to record when people took their medicine, 73 percent claimed that they did it the recommended three times per day. But when the information was downloaded, it showed that only 15 percent were actually taking their medicine as directed.

How it harms you

That such deception can be harmful was underscored by a recent study on osteoporosis treatment. Researchers from the Columbia University Medical Center found that fewer than half of the people in the study took their prescribed medication at all. And these noncompliant patients had a risk of fracture that was 20 percent higher after two years compared with those who followed their medication regimen.

Outright untruths can certainly be damaging, but being less than forthcoming with your doctor also works to your disadvantage. Lipman remembers a 65-year-old graphic designer for whom he prescribed the antidepressant fluoxetine (Prozac and generic versions). Aware of Lipman’s general bias against dietary supplements, the patient did not reveal that he was taking St. John’s wort, a supplement that has shown some efficacy in treating mild to moderate depression. It was weeks before they both realized that the combination of the two was causing the patient to walk around in a fog.

Of course, stopping a medication without informing your doctor can be just as bad as or even worse than not telling him or her the whole story. Lipman recalls the unusual and unfortunate case of a man who abruptly stopped taking his blood pressure medication after he read that it caused cancer. Three days later, he had a stroke.

In Lipman’s experience, the most common reasons for not taking medication are side effects (either feared or real), cost (it’s not covered by insurance) and interference with lifestyle or daily schedules. You can circumvent all of those roadblocks by having a discussion with your doctor. Be sure to talk about the medication’s health benefits vs. the risks, changing medication or adjusting the dosage. Honesty is the best policy, and a lack of it can be dangerous.

Copyright 2015. Consumers Union of United States Inc.

For further guidance, go to www.ConsumerReports.org/Health, where more detailed information, including CR’s ratings of prescription drugs, treatments, hospitals and healthy-living products, is available to subscribers.