In trying to reach an agreement before approval, Dr. Miller said, “this is how the system is supposed to work.”

Christine Cramer, a spokeswoman for CVS Health, another leading pharmacy benefit manager, said, “While we believe our advocacy on behalf of our clients did ultimately influence Regeneron’s initial pricing strategy, the drug will be expensive.”

But Peter Maybarduk, director of the Access to Medicines program at Public Citizen, a consumer group, noted that negotiations still took place out of the public eye and within the context of a system that is inherently unfair. “It’s the arrangement that corporations found beneficial in a system that provides monopolies and rewards secrecy, and we should ask for considerably more as the standard,” he said.

Dupixent treats severe to moderate atopic dermatitis, a common form of eczema that goes beyond the occasional bouts of itchy, dry skin that many people get. For people with serious forms of the disease, other treatments often do little to calm their skin, leading to sleeplessness, depression and social anxiety. Regeneron said Tuesday that an estimated 300,000 people in the United States could qualify for its drug.

“I always say that atopic dermatitis doesn’t kill you, it just ruins your life,” said Dr. Elaine Siegfried, a professor of pediatrics and dermatology at the St. Louis University School of Medicine. She was not involved in the clinical trials that led to approval, but she said she was likely to enroll patients in pediatric studies that are getting underway. Dupixent, she said, appears to work well, with few serious side effects. “It is groundbreaking,” she said.

Debbie Byrnes, a sixth-grade teacher from San Antonio who participated in the clinical trial, had suffered for years with severe eczema that often covered her face. “That was the really difficult thing for me — I could never hide it,” she said. “I would have days when I went into school, and the kids would look at me and say, ‘What happened to you?’ ”

Ms. Byrnes said she began noticing a difference about five days after her first dose and has now been using the drug for two years. Her skin is now almost completely clear, and occasional flare-ups are quickly brought under control. “If you saw me,” she said, “you wouldn’t know that I ever had atopic dermatitis.”