The FDA approved the first Alzheimer’s drug that may really work. A Penn physician explains why you probably can’t get it, yet.

The FDA on Friday authorized the use of the Alzheimer’s drug lecanemab, the first drug clearly shown to slow the progress of the debilitating brain disease.

But the treatment will not be widely available at first, as insurers are not expected to cover it until the agency reviews additional evidence about the drug’s effectiveness.

That review may come later this year. Yet even then, policy experts warn that the health-care system will have trouble accommodating — and paying for — treatment of the millions who will be eligible.

Studies suggest the drug is modestly beneficial for people in the early stages of Alzheimer’s, allowing them to retain more of their memory and thinking skills than people who don’t take the drug. As with any drug, there is a risk of side effects — in this case, swelling and bleeding in the brain.

Here are the key points to keep in mind, with a big assist from neurologist David Wolk, codirector of the Penn Memory Center.

Lecanemab consists of antibodies that bind to abnormal proteins that accumulate in the brains of people with Alzheimer’s disease. That’s what the -ab stands for at the end of the drug’s name.

Once the antibodies latch onto these sticky proteins, called beta amyloid, the immune system removes them from the brain.

The drug is administered via hourlong infusions every two weeks for up to 18 months. Made by Tokyo-based Eisai, it will be sold under the brand name Leqembi.

» READ MORE: Can this drug stop Alzheimer's before it starts?

At first, almost no one — unless they can pay out of pocket.

Last year, in evaluating another costly antibody-based drug called Aduhelm, Medicare ruled it would cover this class of drugs only for patients participating in clinical trials. That’s because Aduhelm had been shown only to reduce levels of abnormal proteins — without any clear impact on outward symptoms.

The agency said it would pay for broader use of such drugs upon reviewing evidence that they actually helped preserve brain function. Indeed, that type of evidence came out for lecanemab several months afterward, in November, but the review will not take place until later this year.

In the meantime, a few private insurers may cover some of the drug’s cost, but it’s unlikely, said Penn’s Wolk.

Or a person could theoretically pay cash for the drug, but it won’t be cheap. Analysts have estimated it will cost $10,000 to $30,000 for a year of treatment.

In a study published in November, people who took the drug for 18 months experienced less decline in brain function than those who took a placebo.

The difference between the two groups was modest. On average, the brain function of those on the drug declined by 1.21 points on an 18-point scale, compared with a decline of 1.66 points for those who did not take it.

But that difference is nevertheless meaningful, said Wolk, who also directs Penn’s Alzheimer’s Disease Research Center. It might be enough to allow people to maintain a fair degree of independence, continuing to perform tasks such as driving and balancing a checkbook.

“Half a point on that scale at a very early stage can be the difference between having minimal issues on memory and brain function to requiring assistance at home,” he said. “There’s good reason to believe that while it’s not a home run, it definitely could impact people in their day-to-day lives.”

People who take lecanemab can experience swelling and bleeding in the brain. In most cases, this effect is so slight that the patient is unaware, Wolk said.

Physicians will monitor patients for signs of these side effects, and can generally manage them with medication.

Because of the risk of brain bleeding, the drug is probably not a good idea for someone who takes blood-thinning medication, he said.

Such safety questions are likely to be closely watched after this month, when the drugmaker reported that one person died after taking the drug. Physicians are of mixed opinion how much, if any, the drug contributed to her death.

For now, the drug is limited to patients with mild cognitive impairment or mild dementia.

Those aren’t the same thing, Wolk said.

People with mild cognitive impairment have some memory or thinking problems, but not enough to impact their day-to-day lives. Mild dementia is a more advanced stage of disease, in which people may be unable to balance their checkbook or remember to take medicines.

People with mild cognitive impairment generally are sharp enough to decide whether the benefits of lecanemab outweigh any risks. Those who are farther along, with mild dementia, may require the guidance of a family member, Wolk said.

“Physicians will have to assess their understanding to make sure,” he said. “We strongly recommend that they have a partner, usually a family member, that helps them in the decision-making process, so that any decision they make would be in keeping with whatever decision they would’ve made prior to having the impairment.”

Once lecanemab is widely available, physicians will be hard-pressed to handle the demand, Wolk predicted.

That’s because in most cases, people will be required to undergo expensive brain scans to confirm that they have a buildup of the abnormal proteins in the brain, at a cost of thousands of dollars apiece.

Someday, blood tests will be a faster, cheaper option once they are perfected, Wolk said. But patients still may have to wait years for the actual drug in some parts of the country, given the shortage of infusion clinics and physicians who are qualified to supervise the treatment.

“It’s going to be a challenge,” Wolk said. “Even in Philadelphia, there’s not enough people in this specialty to take care of all the patients in this region. If you start traveling to other parts of the country, there is maybe one specialist, or maybe none.”