The most widely used drugs are those to lower high blood pressure and cholesterol - problems often linked to heart disease, obesity and diabetes.
The numbers were gathered last year by Medco Health Solutions Inc., which manages prescription benefits for about one in five Americans.
Experts say the data reflect not just worsening public health but better medicines for chronic conditions and more aggressive treatment by doctors. For example, more people are now taking blood pressure and cholesterol-lowering medicines because they need them, said Dr. Daniel W. Jones, president of the American Heart Association.
In addition, there is the pharmaceutical industry's relentless advertising. With those factors unlikely to change, doctors say the proportion of Americans on chronic medications can only grow.
"Unless we do things to change the way we're managing health in this country ... things will get worse instead of getting better," predicted Jones, a heart specialist and dean of the University of Mississippi's medical school.
Americans buy much more medicine per person than any other country. But it was unclear how their prescriptions compare to those of insured people elsewhere. Comparable data were not available for Europe, for instance.
Medco's data show that last year, 51 percent of American children and adults were taking one or more prescription drugs for a chronic condition, up from 50 percent the previous four years and 47 percent in 2001. Most of the drugs are taken daily, although some are needed less often.
The company examined prescription records from 2001 to 2007 of a representative sample of 2.5 million customers, from newborns to the elderly.
Medication use for chronic problems was seen in all demographic groups:
Among seniors, 28 percent of women and nearly 22 percent of men take five or more medicines regularly.
Almost two-thirds of women 20 and older. One in four children and teenagers. 52 percent of adult men. Three out of four people 65 or older.
Karen Walker of Paterson, N.J., takes 18 prescription medicines daily for high blood pressure, diabetes, chronic back and shoulder pain, asthma and the painful muscle disorder fibromyalgia.
"The only way I can do it and keep my sanity ... is I use pill boxes" to organize pills for each morning and night, said Walker, 57, a full-time nurse at an HIV clinic. Her 69-year-old husband, Charles, keeps his medicines lined up on his bureau: four pills for arthritis and heart disease, plus two inhalers for lung problems.
Dr. Robert Epstein, chief medical officer at Franklin Lakes, N.J.-based Medco, said he sees both bad news and good in the findings.
"Honestly, a lot of it is related to obesity," he said. "We've become a couch potato culture (and) it's a lot easier to pop a pill" than to exercise regularly or diet.
On the good side, he said, researchers have turned what used to be fatal diseases into chronic ones, including AIDS, some cancers, hemophilia and sickle-cell disease.
Yet Epstein noted the biggest jump in use of chronic medications was in the 20- to 44-year-old age group - adults in the prime of life - where it rose 20 percent over the six years. That was mainly due to more use of drugs for depression, diabetes, asthma, attention-deficit disorder and seizures.
Antidepressant use in particular jumped among teens and working-age women. Doctors attributed that to more stress in daily life and to family doctors, including pediatricians, being more comfortable prescribing newer antidepressants.
Dr. Sidney Wolfe of Public Citizen's Health Research Group said the increased use of medications is partly because the most heavily advertised drugs are for chronic conditions, so most patients will take them for a long time. He also blames doctors for not spending the time to help patients lose weight and make other healthy changes before writing a prescription.
The study highlights a surge in children's use of medicines to treat weight-related problems and other illnesses previously considered adult problems. Medco estimates about 1.2 million American children now are taking pills for Type 2 diabetes, sleeping troubles and gastrointestinal problems such as heartburn.
"A scarier problem is that body weights are so much higher in children in general, and so we're going to have larger numbers of adults who develop high blood pressure or abnormal cholesterol or diabetes at an earlier age," said Jones, of the heart association.
Dr. Richard Gorman, an American Academy of Pediatrics expert on children's medicines, said more children are taking medicines for "adult conditions" partly because manufacturers now provide pediatric doses, liquid versions or at least information to determine the right amount for a child.
The Medco study found that among boys and girls under age 10, the most widely used medication switched from allergy drugs to asthma medicines between 2001 and 2007. Gorman said that's because over the last decade, asthma care has gone from treating flare-ups to using inhaled steroids regularly to prevent flare-ups and hospitalizations.