Americans who obtained health insurance on exchanges through President Obama's health care law are using more costly prescription drugs than is typical, according to a new report from Express Scripts, the nation's largest provider of pharmacy benefits.

Because claims for prescription drugs tend to come in more quickly than claims for other medical services, the report provides early insights into the composition of enrollees on the new health insurance exchanges. For the exchanges to be viable, they have to have an adequate mix of healthy and sicker participants. Otherwise, insurers will have to hike premiums in future years, discouraging healthier Americans from enrolling. Though the Department of Health and Human Services has given an age breakdown of enrollees through February, that doesn't tell the full story, because it's possible for people to be old and healthy, or young and sick.

"Our early analysis reveals that, in January and February, use of specialty medications was greater among exchange enrollees versus patients enrolled in a commercial health plan," the report found. According to the report, which looked at 650,000 pharmacy claims during the first two months of the year, 1.1 percent of total prescriptions requested through exchange plans were for expensive specialty medications, compared to 0.75 percent in commercial health plans, which represents a 47 percent difference. Though this may seem marginal, the report notes that even when representing less than one percent of prescriptions, specialty drugs represent more than one-fourth of the nation's total spending on pharmaceuticals.

In total, according to the study, "six of the top 10 costliest medications used by Exchange enrollees have been specialty drugs. In commercial health plans, only four of the top 10 costliest medications were specialty."

More specifically, HIV prescriptions were four times as common among exchange enrollees; the proportion of pain medication prescriptions was 35 percent higher; anti-seizure prescriptions were 27 percent more common; antidepressants were 14 percent more common; and the proportion of contraceptives was 31 percent lower.

The fact that enrollees in exchange plans have more demand for prescription drugs is no surprise. The exchanges were set up to help cover individuals who were too sick to afford coverage on the individual market, assuming they were even offered coverage. The fact that exchange enrollees used fewer contraceptives also makes sense, given that they skew older. The relevant question is whether the medical claims end up vastly exceeding what insurers were expecting when they set premium rates — and that will take some more time to determine.

It's also worth noting that this report looked at claims data from January and February, and it's possible that those who held out until March to sign up for coverage were healthier than those who enrolled earlier.