The disease is characterized by an itching, oozing rash that can cover almost all of the skin. The constant itch, to say nothing of the disfigurement, can be so unbearable that many patients consider suicide. There has never been a safe and effective treatment.

On Saturday, the results of two large clinical trials of a new drug offered hope to the estimated 1.6 million adult Americans with an uncontrolled, moderate-to-severe form of the disease, atopic dermatitis, which is a type of eczema.

Most patients who got the active drug, dupilumab, instead of a placebo reported that the itching began to wane within two weeks and was gone in a few months, as their skin began to clear. Nearly 40 percent of participants getting the drug saw all or almost all of their rash disappear.

For some, relief was almost instantaneous.

“I knew immediately I was on the drug” and not the placebo, said Daniela Velasco, an event planner in Playa del Carmen, Mexico. Within a couple of weeks, the ugly red rash that had covered 90 percent of her body was almost gone. Even better, she said, “for the first time I didn’t feel any itch at all.”