As payers continue to foot the bill for orphan drugs, sales of these rare disease treatments are forecast to grow 11 percent over the next five years — to $209 billion. And this is more than twice the expected increase in sales of all other prescription medicines, according to a new analysis.

By 2022, orphan drugs are predicted to account for more than 21 percent of worldwide brand-name prescription drug sales, up from 6 percent in 2000. Last year, sales of orphan drugs, which are used to treat rare diseases for patient populations numbering less than 200,000, climbed more than 12 percent, to $114 billion. By comparison, sales of other brand-name drugs rose 2.4 percent, to $578 billion.