The FDA just approved the drug, baricitinib, manufactured by Eli Lilly & Co., opening up a new course of treatment for hundreds of thousands — possibly millions — of patients. Meanwhile, two similar JAK inhibitor drugs made by other manufacturers, Pfizer and Concert, are in the works.

“It’s hugely important. This is not a tiny amount of people,” said King, an associate professor at the Yale School of Medicine who led two trials, including a yearlong Phase 3 study for the Eli Lilly drug, marketed as Olumiant, which showed significant regrowth of hair among 1,200 patients.

“This is people numbering in the hundreds of thousands, if not 1 or 2 million, in the U.S.” who currently don’t have an effective treatment for their ailment, he said. “So to have the first-ever medicine, FDA approved, will change this.”

Until now, there was “nothing that worked,” said King, who lives in Fairfield. The medicines previously used “were just … immune suppressants,” he said. Now, “we can literally interrupt the messaging between the hair follicles and the immune system,” he said.

Back in 2013, King knew of early research in mice that connected a new class of drugs developed for rheumatoid arthritis, known as Janus kinase inhibitors, or JAK inhibitors, to hair growth. A man in his early 20s, from Killingworth, came to King with psoriasis.

He also had alopecia areata.

King treated the patient, Kyle Rhodes, with a JAK inhibitor made by Pfizer that had been approved for arthritis a year earlier. The results were dramatic — hair regrowth on a balding man with a previously untreatable hair-loss illness.

King published his findings.

The way JAK inhibitors like baricitinib work is by blocking “chemical messages being sent back and forth between hair follicles and immune cells,” King said. “JAK inhibitors literally interrupt the messages … and when those messages are interrupted, the immune cells leave the hair follicles alone.”

The results of the Phase 3 study show that among patients who received a high dose and had at least 50 percent hair loss, 39 percent showed a full or near-full recovery of hair.

King presented the results of the study late March at the American Academy of Dermatologists’ annual meeting. The New England Journal of Medicine published the findings, with King as lead author.

Alopecia areata, a condition that causes dramatic and traumatic hair loss, is different from basic male pattern alopecia, which translates simply as “hair loss,” King said.

It burst into the public consciousness at the Academy Awards ceremony March 27 when actor Will Smith slapped host Chris Rock after Rock made a joke about Smith’s wife, Jada Pinkett Smith’s, lack of hair. Pinkett Smith suffers from alopecia areata.

King’s work had drawn widespread attention for years prior to that, however.

After he first published details of his research in mid-June 2014, “there immediately were hundreds of patients who wanted to come to my clinic,” he said.

One of those patients was Ida Carter of Southington, who now has been a patient of King for nearly two years — and said her experience with baricitinib “has been phenomenal.”

Prior to being treated, “I lost all of my hair everywhere all over my body,” and had suffered with the disease for about 12 years. She was about 33 when her hair loss started, she said.

“For me , this is like super exciting,” Carter said. “Just to have hope” is a huge thing, she said. “There’s a lot of hopeless people with this condition.”

Since she began treatment with baricitinib, “I’ve had almost 100 percent regrowth,” Carter said. “It’s pretty amazing.”