Of the more than 1,600 generic drugs approved by the Food and Drug Administration since January of 2017, more than 700—or 43 percent—are not for sale in the US, according to a new analysis by Kaiser Health News.

The finding means that many pricy, brand-name drugs are not facing the competition that could help drive down soaring prices. Among the drugs missing in action are generic versions of the expensive blood thinner Brilinta and the HIV medication Truvada. Moreover, of the approved drugs that would offer a brand-name drug its first competition, 36 percent are being held off the market, the analysis found.

Experts told KHN that the reasons drug makers may withhold an approved generic from the market are varied. Industry consolidation has made buying, manufacturing, and distributing generics more difficult in recent years. Generic drug makers also, as always, face patent litigation from brand-name makers. Then there’s potentially anti-competitive deals, in which brand-name drug makers simply pay generic makers to keep their product off the market for a while—a so-called “pay for delay” tactic.

Lastly, there are internal decisions within a generic company that can lead to shelving a drug. For instance, a drug maker may shift its business strategy while it’s waiting for the drug to get approved, or the maker may delay a drug’s entry to the market until a strategic time.

Whatever the reason, keeping approved generics from the market is “a real problem because we’re not getting all the expected competition,” FDA Commissioner Scott Gottlieb said in an interview with KHN.

Generic approvals at the FDA have ramped up in recent years, and the agency is cracking down on anti-competitive tactics, Gottlieb said. Still, it’s a difficult problem to solve with so many factors at play, he said.

He added that an FDA analysis found that on average it takes the introduction of five generic versions of a drug to the market to drive down a drug’s price to 33 percent of the original branded price.