Earlier this month, in the wake of Valeant's ($VRX) canceled relationship with controversial specialty pharmacy Philidor, Express Scripts ($ESRX) said it would take a hard look at other specialty pharmacies that deal mostly with a single drugmaker. And now, it's cutting off one that works closely with Ireland's Horizon Pharmaceuticals ($HZNP).

The PBM giant is terminating Linden Care after finding that it "predominantly dispensed Horizon prescription drugs and did not fulfill key components of our pharmacy network agreements," company spokesman Brian Henry toldFiercePharma in an emailed statement. It's also evaluating "several other pharmacies" that seem to be mostly dispensing Horizon's products.

In addition, Express Scripts has sued Horizon itself for breach of contract, looking to recover about $140 million in back payments. The Dublin drugmaker hasn't honored an agreement to pay certain rebates to Express Scripts, the suit says.

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Horizon Pharmaceuticals CEO Timothy Walbert

For Horizon's part, it's still reviewing the allegations in the complaint, it said in a statement, but "we believe the claims are without merit and we intend to vigorously defend ourselves." Horizon's shares dove 18% in early trading Wednesday on the news.

Specialty pharmacies have come into vogue in recent years, because they work with payers on and help link patients with co-pay discounts and other assistance programs. But they came into the spotlight last month, when short-seller Citron Research accused Valeant of using its relationships with them--most notably Philidor--to inflate its top line with "phantom sales."

Valeant, which denied the claims, has since set up an ad hoc committee to investigate them and severed all ties with the pharmacy, citing questionable business practices. Those practices also spurred both Express Scripts and rival CVS Health ($CVS) to drop the pharmacy late last month.

Express Scripts hasn't stopped there, though. It's said it would take a closer look at other "captive pharmacies"--those that derive most of their prescription volume from one drugmaker or one therapy. And while "the notion that Linden Care is a so-called 'captive pharmacy' of Horizon Pharma is entirely false," Horizon said in a statement, Express Scripts isn't convinced.

"We routinely monitor our network and take action when we see pharmacies trying to circumvent solutions, like formulary management, that protect payers and patients from wasteful drug spend," Henry said.

Horizon felt the sting of that formulary management last summer, when Express Scripts and CVS kicked two arthritis pain meds off their formularies. At the time, Horizon predicted the moves could cut the drugs' prescriptions by 20% to 30%. In response, it said it would push prescriptions through its specialty pharmacy program, a strategy it predicted would "mitigate the effects" of the PBMs' decisions.