Medicare Part D premiums are likely to go up next year. Here's why.

Medicare enrollees who buy the optional Part D drug benefit may see substantial premium price hikes — potentially up to $50 a month — when they shop for next year’s coverage.

Such drug plans are used by millions of people who enroll in what is called original Medicare, the classic federal government program that began in 1965 and added a drug benefit only in 2006 . The drug plans are offered through private insurers, and enrollees must pay monthly premiums.

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It’s not known whether insurers will pursue the maximum increase allowed, as premium prices for next year won’t be revealed until closer to open enrollment, which starts Oct. 15.

Increases are expected to mainly affect stand-alone Part D plans, not the dru