The clinical hold comes at an inopportune time for Gilead.

Last month, the company’s longtime rival Merck & Co. disclosed worrisome findings from a study testing a combination of two of its experimental HIV medications. Specifically, it appeared that patients who took the drugs, known as MRK-8507 and islatravir, experienced decreases in certain kinds of white blood cells. Merck subsequently paused work on MRK-8507 while forging ahead with islatravir.

While that setback could have been a competitive gain for Gilead, it proved to be more of a double-edged sword. That’s because earlier in the year, the company agreed to team up with Merck to develop a pairing of lenacapavir and islatravir. Any safety concerns with the latter would therefore have a direct impact on Gilead.

And indeed, not long after Merck’s announcement, the partners chose to pause enrollment in their combination study out of an “abundance of caution.”

Now, the Food and Drug Administration is putting 10 more lenacapavir studies on ice until it’s satisfied that the vial issues have been dealt with.

The main concern, according to Gilead, is that the injectable lenacapavir solution isn’t compatible with vials made of borosilicate glass. The FDA is wary this interaction could cause “sub-visible glass particles” to form in the drug solution.

“We are committed to working diligently with FDA to resolve this glass vial compatibility quality issue and resume injectable lenacapavir dosing in the affected studies in a timely fashion,” Merdad Parsey, Gilead’s chief medical officer, said in the company’s Wednesday statement.

Notably, the islatravir combination study is using an oral rather than injectable version of lenacapavir.

Both drugs are antiviral agents that, in different ways, attempt to prevent HIV from replicating. They’re also closely watched by investors, as Merck and Gilead have, respectively, described the drugs as key to their ambitions in HIV treatment.