With three weeks left for seniors to change their Medicare prescription plan for 2012, a study released Wednesday finds that copays for brand-name drugs are going up _ sharply in some cases.

Copays for preferred brand-name drugs will increase by 40 percent on average next year, and non-preferred brands will average nearly 30 percent more, according to the study by Avalere Health. Copays are the portion of the cost of each prescription that the customer pays the pharmacy.

The report underscores how important it is for Medicare beneficiaries to check their prescription coverage before open enrollment ends Dec. 7.

Medicare announced this summer that premiums for prescription plans would remain unchanged next year, an average of about $30 a month. But the plan with the lowest monthly premium may not always be the best deal.

"Seniors need to look beyond the premium to understand their drug benefit," said Avalere CEO Dan Mendelson. "The more the cost burden gets shifted onto the patient who needs the medication, the more important it is for seniors to understand that next level." Avalare is a data analysis firm serving the health care industry and government.

Medicare officials said they were reviewing the study. Pointing to stable monthly premiums and improved benefits, spokesman Brian Cook said the prescription program is getting stronger.

Mendelson said Avalere's findings show that Medicare prescription plans are steadily shifting costs to patients who need more expensive kinds of medications, while trying to keep costs in check for those whose conditions can be managed with less-expensive generics.

The costly medications include brands and so-called "specialty" drugs for diseases such as multiple sclerosis, cancer and rheumatoid arthritis. By contrast, Avalere found that copays for generic drugs will remain stable or decline.

Medicare covers about 47 million seniors and disabled people, and about 9 in 10 beneficiaries have some kind of prescription drug plan. Most rely on the prescription program, also known as Part D, which is delivered through private insurance plans.

The Avalere study found that copays for preferred brand name drugs will increase to an average of $40.60 next year, up from $29.01 currently. Preferred brands are usually drugs for which the prescription drug plan has negotiated a discount with the manufacturer.

Copays for non-preferred brand drugs will rise to $91.67 on average, from $71.52 this year.

Beneficiaries will also pay a bigger share of the cost of specialty drugs, which can exceed $1,000 or more per prescription. The share for 2012 averages about 32 percent, up from 27 percent this year.

By contrast copays for preferred generics will remain stable, averaging $3.79. And copays for non-preferred generics will drop to $9.90, a 43 percent reduction from the current $17.29.

Medicare prescription plans usually have several levels of coverage _ each with a different level of cost-sharing for the patient. The most common kind of plan has five levels: preferred generics, non-preferred generics, preferred brands, non-preferred brands, and specialty drugs.

Since the Avalere figures are averages for the entire program, actual costs could vary markedly by medication, plan and region of the country.

The study also found big differences in the total number of drugs covered by the top 10 plans. Topping the list is the Humana Enhanced plan, which will cover nearly 80 percent of the more than 2,300 Medicare drugs. By comparison, the WellCare Classic plan will cover just under half.