Sen. Claire McCaskill (D-Mo.) penned a letter to Mallinckrodt Pharmaceuticals (NYSE:MNK) last week, probing for details about the company’s pricing strategies for its injectable pain-killer, Ofirmev.

Mallinckrodt acquired Ofirmev in 2014, when it bought Cadence Pharmaceuticals in a $1.4 billion deal. In May of that year, Mallinckrodt hiked the pain-killer’s list price from $14.75 per vial to $35.40 per vial.

McCaskill pointed to reports showing that the company’s price increases cost hospitals millions, which may have pushed healthcare practitioners to use cheaper painkillers, including highly addictive opioids.

The senator wrote that she is also investigating the actions by top opioid manufacturers to better understand how they market their products.

She requested that Mallinckrodt provide her with quarterly sales data for Ofirmev from before and after the company acquired the injectable acetaminophen formulation. McCaskill also asked for published data showing that Ofirmev is an effective alternative to opioids as a pain management therapy.

Mallinckrodt has been widely criticized for its pricing strategies – 1 of its top-selling products is a multiple sclerosis treatment that comes with a price-tag of more than $30,000 per vial.

But the drugmaker defended itself in response to McCaskill’s probe, saying that the increase to Ofirmev’s list price was to “stabilize” the product’s financial situation and stop it from losing “tens of millions of dollars annually”. It also pointed to customer discount opportunities and investments the company has made to demonstrate the pain-killers’ value.

“These studies reinforce Mallinckrodt’s commitment to providing hospital customers with high-value products that address the clinical needs of patients,” the company wrote.

Mallinckrodt said it has and will continue to cooperate with McCaskill’s investigation.