Drugs to treat H.I.V. and AIDS are being priced out of reach for many patients enrolled in insurance plans through the new health care exchanges, despite warnings that such practices are illegal under the Obama administration’s health care law, according to a new analysis by Harvard researchers.

The study, to be published on Wednesday in an article in The New England Journal of Medicine, looked at 48 health plans in 12 states and found that a quarter of the plans showed evidence of what researchers called “adverse tiering,” or placing all of the drugs used to treat H.I.V. in a specialty tier where consumers are required to pay at least 30 percent of the cost of the drug.

The financial impact can be drastic, the researchers found: A patient taking a common H.I.V. treatment, Atripla, would pay about $3,000 more a year in a restrictive plan compared with someone enrolled in a more generous plan, even after accounting for the fact that the more restrictive plans tended to charge lower monthly premiums.

“That’s really a large cost difference, and really is a very significant financial constraint for those with chronic conditions, particularly H.I.V.,” said Douglas B. Jacobs, the lead author of the study, who is pursuing degrees in public health at the Harvard T. H. Chan School of Public Health and medicine at the University of California, San Francisco.

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