The PBMs—which act as middlemen between drugmakers, health insurers and pharmacies—counter that the administrative process the agency is using to bring its claims is unconstitutional. Further, they blame drugmakers for the high cost of insulins and say they actually save consumers money by lowering net drug costs. They also argue that some clients negotiate rebates and create drug formularies that PBMs merely administer.

The stakes of the case are high, Proskauer Rose wrote in a note to clients. "The FTC's case, though targeted on a single drug, attacks the core of PBMs' business models and a central aspect of drug pricing," the firm attorneys wrote. "Follow-on litigation can be expected. It will be watched closely, as potential plaintiffs look at whether rulings on rebating practices for insulin may be repeated and expended to other drugs."

So far, the companies targeted for records production have been successful in obtaining extensions from the FTC's administrative law judge, without opposition from PBMs or the commission.

Mercer on Tuesday even asked the FTC judge for a third extension of time.

"Granting the proposed extension will allow counsel to continue negotiating in good faith in an effort to narrow or limit any disputes," Mercer attorney Timothy McDonald of Thompson Hine wrote in the petition.

Mercer's extension involves subpoenas served from Caremark Rx, Express Scripts and their group purchasing affiliates.

The non-parties say the records requests are becoming burdensome.

In its Jan. 31 filing, McDonald's said that after it it received a subpoena from Caremark, it suggested that the PBM review the fast food giant's responses to the FTC's subpoena and then meet with its attorneys.

Then came a subpoena from Express Scripts and its affiliates.

McDonald's said it suggested taking the same approach with Express Scripts.

"This properly balances the burden placed on non-party McDonald's with the needs of the parties," the chain's outside counsel, Aronberg Goldgehn Davis & Garmisa, wrote in its second request for an extension to produce documents.

"Respondents are never shy about seeking information from third parties they think will be helpful. I'm not surprised that non-parties served with subpoenas would push back, at least requesting additional time to respond," said John Lopatka, a law professor at Penn State University and former assistant director of the FTC's Bureau of Competition.

"Just how difficult it is to respond is a function of the scope of a subpoena and the nature of the information requested. And in a market involving health care and PBMs, I would imagine the effort required to respond is tremendous," Lopatka added.

Related: The state of PBM regulation in the states

The FTC faces a delicate dance in working with non-parties. It wants to obtain potentially valuable evidence ASAP but not push so hard as to make its interactions with non-parties contentious. While third parties have to comply with the subpoenas, they can push back in a variety of ways, such as arguing the request is too broad, calls for the disclosure of confidential information or would be unduly burdensome to fulfill.

In some cases, the nonparty holds a trove of information that may not be available from another source. For example, in late December Express Scripts won approval to subpoena the nonparty Department of Defense partly on the grounds that the documents it sought were "uniquely within DOD's custody" and purported to show that it was DOD, rather than Express Scripts, that negotiated its rebates and created its formularies.

The FTC's negotiations with nonparty insulin-makers Eli Lilly, Novo Nordisk and Sanofi Adventis are especially sensitive, given the right jab the companies absorbed in the FTC's press release announcing its case against the PBMs.

The Bureau of Competition "remains deeply troubled by the role drug manufacturers like Eli Lilly, Novo Nordisk and Sanofi play in driving up list prices of life-saving medications like insulin," the release said. "Indeed, all drug manufacturers should be on notice that their participation in the type of conduct challenged here raises serious concerns, and that the Bureau of Competition may recommend suing drug manufacturers in any future enforcement actions."

"It will be interesting to see the impact of this rhetoric," David Schwartz, a partner at Bryan Cave Leighton Paisner, said in a client note on the case.

"A successful working relationship with third parties is crucial for any FTC action, and the FTC's decision to make their concerns public while foregoing suit against the manufacturers creates a challenge for its staff to foster a cooperative relationship for integral third parties," he wrote.

University of Michigan associate law professor Christopher Knight, who formerly practiced at Mayer Brown and at Miller Canfield, recalled receiving subpoenas while working as outside counsel for an automotive company.

"The best way to get on my good side was to have a really, really narrow scope. Asking for everything under the sun is a good way to rub in-house counsel the wrong way," Knight said.

Before trying to figure out who in the company had the information, he'd call counsel of the demanding party and say, "Why don't you just tell me exactly what you need?"

All too often subpoenas wound up too broad. Knight said that many attorneys never worked in-house. Or, it was, "Hey, third-year, fourth-year associate—I need you to draft a subpoena."

Often, non-parties find responding to subpoenas burdensome because of the time required to track down and review documents, he said, rather than because of the costs associated with doing so.

"The art is understanding the work that you are asking the company to undertake, he said.