The United States Food and Drug Administration on Friday approved a new Alzheimer's drug for patients in the earliest stages of the mind-robbing disease – the first of its kind with clear-cut evidence that it can slow, by several months, the disease.

It's a long-needed new treatment, but experts also are voicing a lot of caution: The drug isn't a cure, it's only intended for early-stage patients, requires IV doses every two weeks, and comes with some safety concerns.

It's unclear just how noticeable that modest benefit will be in people's everyday lives.

Still, "it's a landmark," said Dr. Richard Hodes, director of the National Institute on Aging. "It's not enough, but it's encouraging that there's something we can do."

Here are some things to know about the Food and Drug Administration's approval of lecanemab, to be sold under the brand name Leqembi:

How does it work?

The drug, made by Japan's Eisai and its U.S. partner Biogen, is designed to target and clear away a sticky protein called beta-amyloid that builds up into brain-clogging plaques – one key hallmark of Alzheimer's disease.

It targets a slightly different form of that amyloid, possibly explaining why it proved successful in a rigorous study. At the same time, a long list of prior amyloid-targeting drugs has failed, said Dr. Sam Gandy, an Alzheimer's expert at New York's Mount Sinai Hospital.

The FDA cleared a similar drug named Aduhelm in 2021 even though studies never proved it helped patients, a move that triggered scathing criticism from a congressional investigation.

How effective is it?

In Eisai's 18-month study of nearly 1,800 people, Leqembi appeared to delay early-stage patients from worsening by about five months.

That was measured on an 18-point scale that tracks cognitive and functional abilities. People who got the drug still worsened but not as rapidly as those given dummy versions – a difference of nearly half a point on that scale by the study's end.

Experts are divided over how meaningful a benefit is. In addition, it may be hard for families to tell if a loved one's decline has slowed, Gandy said.

Other experts say slowing the disease early on, when people still function well, is essential even if it's not that easy to spot.

"Several months with better cognition, what's that worth to you?" Hodes asked. "I think you can get a strong argument: If I can interact with my family, be independent for months ... that's a significant outcome."

What are the risks?

Like other amyloid-targeting medications, Leqembi can cause brain swelling or small brain bleeds. In Eisai's study, 13% of drug recipients had swelling and 17% had small brain bleeds.

The likely reason: Amyloid plaques usually form around nerve cells in the brain, but sometimes the gunk gets inside blood vessels, too. Gandy explained that pulling amyloid out of those blood vessels can weaken them, making them leaky.

While the brain swelling and bleeds may cause only minimal symptoms such as dizziness and vision problems, they occasionally can be severe - and several Leqembi users have died while taking the drug, including two who were on blood-thinning medications.

Eisai has said the deaths couldn't be attributed to its Alzheimer's drug. But Gandy noted the most significant risk of severe bleeding would be among Leqembi users who also take blood thinners, commonly used by older adults to prevent or treat strokes.

Patients also may experience temporary reactions after the infusions, including fever, flu-like chills, nausea, and blood pressure fluctuations.

Who is eligible?

The drug is only intended for people in the early stages of Alzheimer's who also have that tell-tale amyloid buildup.

The FDA warned doctors to use caution when prescribing Leqembi to people who use blood thinners. Patients also will be urged to get brain scans several times throughout treatment.

As patients gradually worsen, it's unclear how long they should continue getting the twice-a-month IV infusions. So study participants are being tracked for extended periods, and other research is exploring even earlier use – before people with amyloid buildup show symptoms.

When is it available?

Eisai says the drug should be available by Jan. 23, but most patients will likely have to wait months to get it.

That's because health insurers are expected to scrutinize the drug's effectiveness before deciding whether to cover it, for which patients, and what testing they may require to confirm they are good candidates.

And Medicare, which covers most people with Alzheimer's, is not expected to pay for the drug until later this year. That's because the plan for seniors only pays for Alzheimer's drugs that have received full approval from the FDA – while the agency granted Leqembi's approval using a shortcut based on preliminary study results. The FDA is set to review that more extensive 18-month study soon, anticipating full support later this year.

How much is cost?

The IV drug will cost about $26,500 for a typical year's treatment. If insurers cover it, most people won't pay anywhere near that much – although people with insurance that requires them to shoulder a more significant portion of their drug costs could pay thousands a year.