More Americans than ever are on some sort of prescription drug, with increases seen in almost every variety of medications treating many medical conditions, a study says.

From 2000 to 2012 the percentage of U.S. adults using prescription drugs rose from 51 percent to 60 percent, and in 2012, 15 percent of adults were taking five or more medications, almost double the percentage in 2000, researchers found.

Significant increases were seen in the number of people taking cholesterol-lowering drugs, high blood pressure medications and antidepressants, they report in the Journal of the American Medical Association.

Substantial increases in prescription drug use were seen in people 40 to 64 years of age as well as those 65 years and older, the researchers noted, but not among adults 20 to 39 years old.

They pointed to obesity as a major factor driving the rise in prescription drug use.

"Eight of the 10 most commonly used drugs in 2011-2012 are used to treat components of the cardiometabolic syndrome, including hypertension, diabetes, and dyslipidemia," the study authors wrote.

Dyslipidemia is an excessive amount of cholesterol and/or fats in a person's blood.

Drugs to treat gastroesophageal reflux, a condition common in overweight or obese people, is also frequently prescribed and used more and more, the researchers point out.

"Thus, the increase in use of some agents may reflect the growing need for treatment of complications associated with the increase in overweight and obesity," the study authors say.

While most types of prescription drugs are seeing increased use, researchers aren't exactly sure why some kinds are increasing while others aren't.

Increases in one class of drug may mean more people are getting treated for a particular condition, or it may be signs of a change in the underlying health needs of the population, says lead researcher Elizabeth Kantor.

"We wanted to create a comprehensive resource on prescription drug use among U.S. adults, and I think that the implications of these trends vary across classes of drugs," Kantor says of the study, done when she was at the Harvard School of Public Health.

The most commonly used individual drug in 2011-2012 was simvastatin, followed by lisinopril, levothyroxine, metoprolol, metformin, hydrochlorothiazide, omeprazole, amlodipine, atorvastatin and albuterol.

Dr. David Katz, head of the Yale University Prevention Research Center, acknowledges that increase in prescription drug use is bringing a decline in the U.S. death rate, with the drugs proving effective in treating many diseases.

"But, of course, not dying isn't the same as truly living, and that leads to the very ominous implications of this trend," he says, noting that many drugs are prescribed for conditions that could be prevented by a healthier lifestyle.

"Consider the irony. Here in the U.S., we aggressively peddle foods that propagate illness, and drugs to treat the illness that ensues. Big Food and Big Pharma are the winners — we and our families, the losers," he says.