While higher copayments often limit the frequency with which patients use many traditional medications, cost-sharing has relatively little impact on the use of expensive new "specialty pharmacy" drugs.

That finding, being published today in the policy journal Health Affairs, is important because health plans are struggling to pay for the drugs, which can cost as much as $10,000 per month.

Copayments have been effective in controlling other pharmacy costs, it said, but not specialty pharmacy drugs that include injectable and biological agents that target a gene or protein, and in many cases treat complex chronic conditions such as anemia, cancer and multiple sclerosis.

Specialty drugs often are found in a so-called "fourth tier" of health plan formularies, meaning they are subject to different copayments than traditional drugs in the first three tiers of drugs covered by the plan.

A health plan that might charge $5, $20 and $40, respectively, for generic, preferred-brand and nonpreferred brand drugs, for example, may require beneficiaries to pay 25 percent co-insurance for high-cost drugs, with a maximum out-of-pocket expense of $1,000 per year.

Even so, the study found that copayments only cut use of specialty drugs by 1 percent to 21 percent, vs. 30 percent to 50 percent for traditional medicines. That's one reason why health plans should just cover the super-sized cost of the new drugs, say Rand researchers, who note that copayments for specialty drugs can create extreme financial challenges for patients.

"Patients are desperate for these drugs, and they're willing to pay quite a bit to try whatever is out there," said Dana Goldman, chair and director for health economics at Rand, which led the research project. "Even though they're expensive, that cost is spread over a very large insurance pool, so, socially, it makes sense that we should cover them," he said.

While health plans might be tempted to shift specialty drug costs to patients, they shouldn't do so, Mr. Goldman said, because the medications represent just the sort of medical need for which people should buy coverage.

"Insurance markets work best when there is the chance of substantial loss, when that loss is sufficiently rare and uncertain, and when the presence of coverage will not alter behavior much," the study's authors wrote. "Viewed this way, specialty drugs appear to warrant greater, not less, coverage than traditional pharmaceutical agents."

Specialty pharmacy drugs are more costly in part because they often are more difficult to manufacture than traditional medicines, said Mr. Goldman, and the monthly tab for specialty treatments is much higher.

Whereas a 30-day supply of a drug to treat high cholesterol might cost $70 to $100, Mr. Goldman estimated, specialty drugs can cost several thousand dollars for just one monthly treatment. The study found that spending in 2004 for Enbrel, a treatment for rheumatoid arthritis and psoriasis, was $16 million, or about $10,000 per patient.

Another reason specialty drugs carry huge price tags is the relatively small patient population that uses them, Mr. Goldman said.

In Pittsburgh, the growing use of specialty pharmacy drugs prompted Highmark Inc., the region's largest health insurer, to create a company called Medmark that fills specialty prescriptions and manages care for patients. This summer, Highmark sold its remaining stake in Medmark to pharmacy giant Walgreen Co.

Separately, executives at Canonsburg-based Mylan Laboratories have talked openly about their hope that the Food and Drug Administration will allow generic drug companies to manufacture low-cost versions of specialty pharmacy drugs.

Mylan's purchase last month of an Indian company that manufactures drug ingredients was seen as a significant step in this direction since some companies in India already manufacture generic versions of specialty drugs.