Nearly 70 percent of Americans are on at least one prescription drug, and more than half take two, Mayo Clinic researchers say. Antibiotics, antidepressants and painkilling opioids are most commonly prescribed, their study found. Twenty percent of patients are on five or more prescription medications, according to the findings, published online in the journal Mayo Clinic Proceedings.

Researchers find the data valuable because it gives insight into prescribing practices. The statistics from the Rochester Epidemiology Project in Olmsted County, Minn. are comparable to those elsewhere in the United States, says study author Jennifer St. Sauver, Ph.D., a member of the Mayo Clinic Population Health Program in the Mayo Clinic Center for the Science of Health Care Delivery.

"Often when people talk about health conditions they're talking about chronic conditions such as heart disease or diabetes," Dr. St. Sauver says. "However, the second most common prescription was for antidepressants - that suggests mental health is a huge issue and is something we should focus on. And the third most common drugs were opioids, which is a bit concerning considering their addicting nature."

Seventeen percent of those studied were prescribed antibiotics, 13 percent were taking antidepressants and 13 percent were on opioids. Drugs to control high blood pressure came in fourth (11 percent) and vaccines were fifth (11 percent). Drugs were prescribed to both men and women across all age groups, except high blood pressure drugs, which were seldom used before age 30.

Overall, women and older adults receive more prescriptions. Vaccines, antibiotics and anti-asthma drugs are most commonly prescribed in people younger than 19. Antidepressants and opioids are most common among young and middle-aged adults. Cardiovascular drugs are most commonly prescribed in older adults. Women receive more prescriptions than men across several drug groups, especially antidepressants: Nearly 1 in 4 women ages 50-64 are on an antidepressant.

For several drug groups, use increases with advancing age.

"As you get older you tend to get more prescriptions, and women tend to get more prescriptions than men," Dr. St. Sauver says.

Prescription drug use has increased steadily in the U.S. for the past decade. The percentage of people who took at least one prescription drug in the past month increased from 44 percent in 1999-2000 to 48 percent in 2007-08. Spending on prescription drugs reached $250 billion in 2009 the year studied, and accounted for 12 percent of total personal health care expenditures. Drug-related spending is expected to continue to grow in the coming years, the researchers say.