There's big news in the fight against Alzheimer's disease: The first drug that's been proven to slow the progression of the devastating type of dementia just received full approval from the U.S. Food and Drug Administration (FDA). Lecanemab (which is sold under the brand name Leqembi) helps reduce amyloid plaques in the brain, which are hallmarks of Alzheimer's disease. The drug was approved in January under the FDA's Accelerated Approval pathway, which allows the organization to approve drugs for serious conditions where there is an unmet medical need, based on clinical data demonstrating the drug's effect on a surrogate endpoint. But the FDA required a clinical trial to verify the benefit and efficacy of Lecanemab."Today's action is the first verification that a drug targeting the underlying disease process of Alzheimer’s disease has shown clinical benefit in this devastating disease," Teresa Buracchio, acting director of the Office of Neuroscience in the FDA's Center for Drug Evaluation and Research, said in a statement on Thursday. "This confirmatory study verified that it is a safe and effective treatment for patients with Alzheimer’s disease."The approval won't just open up the drug to people who need it – it may also be affordable. The Centers for Medicare and Medicaid Services said Thursday that it will include Lecanemab in its coverage. This is big news, as without coverage, the annual treatment cost could be more than $26,000. Alzheimer's disease impacts nearly 6 million Americans, and more are expected to be diagnosed with the condition in the future, the Centers for Disease Control and Prevention (CDC) says. It's also the sixth leading cause of death in the country. There is currently no cure for the disease, but experts say having this new treatment can help patients have a better quality of life. With the news, it's understandable to have questions about what Lecanemab is and how it works. Here's what you need to know. What is Lecanemab (Leqembi)?Lecanemab (a.k.a. Leqembi) is the first amyloid beta-directed antibody to be converted from an accelerated approval to a traditional approval for the treatment of Alzheimer's disease, the FDA explains. The drug works by reducing amyloid plaques that form in the brain, which is a hallmark of Alzheimer's disease. With this, it helps slow the progression of the disease. Lecanemab is given to patients via an IV infusion. How effective is it?There have been several studies on Lecanemab and its effectiveness. Results of a phase 3 clinical trial conducted at 235 different medical sites across North America, Europe, and Asia, that were published in the New England Journal of Medicine found that Lecanemab reduced cognitive and functional decline in patients with Alzheimer’s disease by 27%. It also found that patients who took the medication had more amyloid plaques cleared after 18 months on the drug. The trial, which included 1,795 adults between the ages of 50 and 90 who had mild cognitive impairment due to early Alzheimer’s disease or mild Alzheimer’s disease-related dementia, grouped participants into those who were assigned to receive Lecanemab via IV infusion every two weeks or a placebo.Participants in both groups had a clinical dementia rating of around 3.2 when the trial began. At the end of the 18-month trial, the score rose 1.2 points in the group that received Lecanemab, but it went up nearly 1.7 points in the placebo group. (Note: A higher score means someone has more cognitive impairment.)Video above: Expert says new drug can slow Alzheimer's progressionAmyloid levels were tracked, too. The average amyloid level was 77.92 centiloids in the Lecanemab group and 75.03 centiloids in the placebo group. At the end of the trial, the average levels dropped to 55.48 centiloids in the Lecanemab group, and went up 3.64 centiloids in the placebo group.The latest study – called Study 301 – was a multi-center, randomized, double-blind, placebo-controlled, parallel-group study that enrolled 1,795 patients with Alzheimer’s disease. Patients were either randomly assigned to receive 10 milligrams of Lecanemab every two weeks or a placebo. The FDA says that Lecanemab "demonstrated a statistically significant and clinically meaningful reduction of decline from baseline to 18 months on the primary endpoint" compared to a placebo. Side effectsIt's important to note that Lecanemab isn't perfect."There are major risks around bleeding and brain swelling," says Amit Sachdev, M.D., medical director for neurology and ophthalmology at Michigan State University and the principle investigator for the Lecanemab clinical trial program at Michigan State University. (Dr. Sachdev has cared for patients taking this medication since 2019.) "These risks need to be very carefully considered," he adds. The latest study found that the following were the most common side effects of Lecanemab: Headache Infusion-related reactions Amyloid-related imaging abnormalities (ARIA)The FDA points out that ARIA most commonly shows up as temporary swelling in areas of the brain seen on imaging studies that usually resolves over time and may happen alongside small spots of bleeding in or on the surface of the brain. ARIA may also come with serious and life-threatening brain edema (swelling in the brain) that can be linked with seizures and other severe neurological symptoms. Intracerebral hemorrhages (i.e. bleeding in the brain) can happen and may be fatal. As a result, a boxed warning is included in the prescribing information.The drug's phase 3 clinical trial found that Lecanemab caused infusion-related reactions in more than 26% of patients and ARIA in 12.6% of patients. More than 17% of people in the Lecanemab group had brain bleeding (compared to 9% of those in the placebo group). "Patients will need solid evidence that they have Alzheimer's and need to have easy access to emergency care if they chose to go on the drug," Dr. Sachdev says. "Brain swelling and bleeding are very serious side effects and minor neurologic symptoms such as dizziness and headache could be due to such swelling."How much will Lecanemab (Leqembi) cost?It's not clear at the moment. However, Medicare and Medicaid covering the drug indicates that it should become affordable – or relatively affordable – for a large range of people. Without coverage, the annual treatment cost could be more than $26,000. What does FDA approval mean?This is the first medication proven to slow the progression of Alzheimer's disease, making it a revolutionary treatment. Its approval is also a big deal."Full approval with the FDA is very difficult to achieve," Dr. Sachdev says. "It is the gold standard for a treatment." If a medication is experimental, there is no expectation that insurance will cover it, he points out. But now that it’s approved, it's "at least possible" that insurance providers will consider covering Lecanemab, he says. The actual impact on patients can vary. "For those patients who are later in their disease, it may not have a big impact," Dr. Sachdev says. "For patients earlier in their disease, this could be a useful tool for slowing progression."Now that the medication is approved, it's important for patients to talk to their doctors, says David Merrill, M.D., Ph.D., a geriatric psychiatrist and director of the Pacific Neuroscience Institute’s Pacific Brain Health Center in Santa Monica, CA. "This drug is complicated," he says. "For some, it can slow down how quickly they get worse, but it comes with risks. It's not a simple decision. It needs to be a shared decision."Video above: Doctor aims to reduce impact of Alzheimer's on Black communitiesDr. Merrill says that there are "still a lot of questions in terms of who will benefit from this treatment and what will help people improve."Paul Newhouse, M.D., clinical core director of the Vanderbilt Alzheimer’s Disease Research Center, says it may take a few months before Lecanemab is able to be administered at major medical centers. "It's going to take us a little bit of time to set up a procedure to get this administered – there are many steps involved," he says. "We have to properly screen patients, get patients genetically screened, and then set up the infrastructure to properly administer this. We also have to confirm that the patient has high levels of beta-amyloid in the brain before we start administering this treatment."While Lecanemab comes with the risk of potentially serious side effects, Dr. Sachdev says he's "excited" for patients and their families. "The medication is far from perfect, but having a chance to have a discussion is remarkable," he says.