The Food and Drug Administration on Thursday approved an innovative new treatment for patients with a form of lung cancer. It is to be used only by patients who have exhausted all other options to treat small cell lung cancer, and have a life expectancy of four to five months.

The drug tarlatamab, or Imdelltra, made by the company Amgen, tripled patients’ life expectancy, giving them a median survival of 14 months after they took the drug. Forty percent of those who got the drug responded.

After decades with no real advances in treatments for small cell lung cancer, tarlatamab offers the first real hope, said Dr. Anish Thomas, a lung cancer specialist at the federal National Cancer Institute who was not involved in the trial.

“I feel it’s a light after a long time,” he added.

Dr. Timothy Burns, a lung cancer specialist at the University of Pittsburgh, said that the drug “will be practice-changing.”

(Dr. Burns was not an investigator in the study but has served on an Amgen advisory committee for a different drug.)

The drug, though, has a side effect that can be serious — cytokine release syndrome. It’s an overreaction of the immune system that can result in symptoms like a rash, a rapid heartbeat and low blood pressure.

Each year, about 35,000 Americans are diagnosed with small cell lung cancer and face a grim prognosis. The cancer usually has spread beyond the lung by the time it is detected.

The standard treatment is old-fashioned chemotherapy — unchanged for decades — combined with immunotherapies that add about two months to patients’ life span. But, almost inevitably, the cancer resists the treatment.

“Ninety-five percent of the time it will come back, often in a matter of months,” Dr. Burns said. And when it comes back, he added, patients find it harder to tolerate the chemotherapy, and the chemotherapy is even less effective.

Most patients live just eight to 13 months after their diagnosis, despite having chemotherapy and immunotherapy. The group of patients in the clinical trial had already had two or even three rounds of chemotherapy, which is why their life expectancy without the drug was so short.

The dismal prognosis for small cell lung cancer is in sharp contrast to the situation with the other, more common non-small cell lung cancer, which has been a triumph of the revolution in cancer treatments. New targeted therapies seek out molecules those cancers need to grow, containing their spread.

As a result, Dr. Thomas said, many patients with that form of lung cancer live so long that their illness becomes “almost like a chronic disease.”

There were several reasons that patients with small cell lung cancer had been left behind.

One is the type of gene mutation the cancer relies on to grow.

Dr. Jay Bradner, Amgen’s chief scientific officer, explained that other cancers are caused by aberrant genes that are turned on. Treatment involves drugs to turn those genes off.

But small cell lung cancer is propelled by genes that are turned off, which makes them difficult to target, Dr. Bradner explained. Another reason is the cancer’s ability to block immune system cells that try to destroy it.

Tarlatamab is an antibody built to overcome those obstacles. It has two arms, the first of which latches onto the growth-promoting molecule that sticks up like a flag from the surface of the cancer cells. It serves as an identification tag for the drug, allowing tarlatamab to find the cancer cells. The other arm grabs a T cell floating by in the bloodstream. The T cell, a white blood cell, can kill cancers if it can get close to them.

The drug brings the T cell and the cancer cell together, poking holes in the cancer or activating genes that make it self-destruct.

Patients in the clinical trial say they have gotten their lives back.

Martha Warren, 65, of Westerly, R.I., found out last year that she had small cell lung cancer. She joined Facebook groups and immediately saw the bad news — most patients do not live long. Her best hope, she decided, was a clinical trial. After chemotherapy and immunotherapy, with her cancer growing rapidly, she was accepted into the Amgen study and began going to Yale for infusions of the drug.

Almost immediately her cancer began shrinking — dramatically.

“I feel as normal as I did before I had cancer,” Ms. Warren said. “There’s a lot of hope with this drug,” she added.

The Amgen study, and the approval, though, involved patients like Ms. Warren who had already gone through a couple of rounds of treatment. Could tarlatamab help earlier?

Amgen is starting such a study now, testing the drug right after initial chemotherapy.