Almost 70% of Americans are taking at least one prescription drug, more than half take two, and 20% are on five or more, according to new research.

The most commonly-prescribed drugs are antibiotics, but antidepressants come second, which suggests that "mental health is a huge issue and is something we should focus on," says study co-co-author Jennifer St Sauver, of the Mayo Clinic Center for the Science of Health Care Delivery.

And the fact that painkilling opioids are the third most commonly-prescribed drugs in the US "is a bit concerning considering their addicting nature," she adds.

17% of the people studied were being prescribed antibiotics, 13% were taking antidepressants and 13% were on opioids, say the study findings, which are reported in the journal Mayo Clinic Proceedings. Lipid-lowerers came in fourth (11%) and vaccines were fifth (also 11%). Drugs were prescribed to both men and women across all age groups, except those to treat high blood pressure, which were seldom used before age 30.

Overall, women and older adults receive more prescriptions, the research shows. Vaccines, antibiotics and anti-asthma drugs are most commonly prescribed in people younger than age 19, while antidepressants and opioids are most common among young and middle-aged adults, and cardiovascular drugs are most widely-prescribed in older adults. 

Women receive more prescriptions than men across several drug groups, especially antidepressants - nearly one in four women aged 50-64 are on an antidepressant.

Prescription drug use has increased steadily in the US for the past decade, the authors note. The percentage of people who took at least one prescription drug in the past month increased from 44% in 1999-2000 to 48% in 2007-8. 

Meantime, a new government report has found  "questionable" prescribing patterns among 736 physicians who prescribe drugs under the federal Medicare health programme's prescription drug benefit, known as Part D, and has called for an investigation.

The Office of the Inspector General (OIG) at the Department of Health and Human Services (HHS) says it conducted the report because concerns about Medicare fraud, and particularly prescriber fraud, have increased along with the rise in prescription drug abuse. 

In 2009, more than one million individual prescribers ordered drugs paid for by Medicare Part D. The IOG report focused on the prescribing of almost 87,000 general-care physicians during that year and found that, for 736 of them, their prescribing patterns marked them as "extreme outliers."

Many of the 736 were prescribing extremely high numbers of prescriptions per beneficiary, which may indicate that these prescriptions are medically unnecessary, the report suggests. And more than half of them had ordered extremely high percentages of Schedule II or III drugs, which have potential for addiction and abuse. "Although some of this prescribing may be appropriate, such questionable patterns warrant further scrutiny," it notes.

These findings show the need for increased oversight of Part D, says the OIG. It makes a number of recommendations, including following-up on the "extreme outliers," whose prescribing cost Medicare $352 million in 2009.

The public-interest newsroom ProPublica notes that the IOG's findings mirror those of its own investigation in May, which found that Medicare has "failed to protect patients from doctors and other health professionals who prescribed large quantities of potentially harmful, disorienting or addictive drugs."

• Healthcare inflation in the US will dip to "a historic" 6.5% in 2014, defying general post-recession patterns, and is likely to be sustained even as millions more newly-insured Americans join the system from next year, forecast a new report from PricewaterhouseCooper (PwC)'s Health Research Institute (HRI). 

This decline in spending growth will present financial challenges for the industry as it attempts to navigate in a rapidly-changing environment, the report warns.