Factors other than the economy that may also be at play in the prescription downturn include adverse publicity about some big-selling medications  like the cholesterol medications Zetia and Vytorin, marketed jointly by Merck and Schering-Plough. And sales of Zyrtec, a popular allergy medication, moved out of the prescription category earlier this year when Johnson & Johnson began selling it as an over-the-counter medication.

Diane M. Conmy, the director of market insights for IMS Health, said the drop in prescriptions might also be partly related to the higher out-of-pocket drug co-payments that insurers are asking consumers to pay.

“Some consumers are making decisions based on the fact that they are bearing more of the cost of medicines than they have in the past,” Ms. Conmy said.

The average co-payment for drugs on insurers’ “preferred” lists rose to $25 in 2007, from $15 in 2000, according to the Kaiser Family Foundation, a nonprofit health care research organization. And, of course, lots of people have no drug insurance at all. That includes the estimated 47 million people in the United States with no form of health coverage, but it is also true for some people who have medical insurance that does not include drug coverage  a number for which no good data may exist.

For older Americans, the addition of Medicare drug coverage in 2006 through the Part D program has meant that 90 percent of Medicare-age people now have drug insurance. And in the early going, Part D had helped stimulate growth in the nation’s overall number of prescriptions, as patients who previously had no coverage flocked to Part D.

But a potential coverage gap in each recipient’s benefit each year  the so-called Part D doughnut hole  means that many Medicare patients are without coverage for part of the year.

The recent IMS Health figures reveal that prescription volume declined in June, in July and again in August, mirroring studies from last year suggesting that prescription use begins dropping at about the time more Medicare beneficiaries begin entering the doughnut hole.