NEW YORK (Reuters) - Some insurers selling policies under Obamacare may be structuring drug coverage in a way that dissuades people with HIV-AIDS from becoming their customers, according to a study published on Wednesday in the New England Journal of Medicine. President Barack Obama's 2010 Affordable Care Act outlawed discrimination based on pre-existing conditions. Insurers cannot ask about current or past illnesses, charge people with, say, diabetes or cancer more than healthy people, or deny coverage. Some, however, have found a way to discriminate anyway, wrote researchers at the Harvard T.H. Chan School of Public Health, "resorting to other tactics to dissuade high-cost patients" from enrolling. Traditionally, insurers use formularies, lists of covered drugs and their out-of-pocket costs, to steer patients toward generic drugs or medicine for which a company has negotiated a favorable price. But formularies can be structured to put off people with pre-existing conditions, said co-author Ben Sommers. He and Douglas Jacobs analyzed 48 Obamacare policies in 12 states using the federal Healthcare.gov marketplace. In 2014, 12 of the policies placed all covered HIV drugs called nucleoside reverse-transcriptase inhibitors, including generics, in tiers with at least a 30 percent co-pay, usually the highest. Some did not cover the drugs at all. Customers in the 12 plans had an average annual cost per HIV drug more than triple that of customers in the other plans, $4,892 compared with $1,615. Premiums in plans with the highest out-of-pocket costs for HIV drugs were lower than others', but someone with HIV would pay $3,000 more each year, the researchers calculated. "This matches what we've seen in Illinois, where four of seven plans have what we consider unaffordable HIV drugs," said John Peller, president of the AIDS Foundation of Chicago. "We think insurers are looking for ways to make plans less welcoming to people with chronic conditions." Insurers disagree. "Individuals can pick the policy that is best for them," said Clare Krusing, spokeswoman for America's Health Insurance Plans, the industry's lobbying group. Subsidies "reduce out-of-pocket costs for prescription drugs even more." In addition, Obamacare requires that insurers with few costly patients pay those with many, so deterring HIV customers makes no financial sense, she said. Most of that transfer program phases out after 2016, however. In late 2014, the Obama administration proposed a rule that would consider policies with all drugs for a condition in the highest out-of-pocket tier discriminatory and therefore disallowed.