Good afternoon! The launch of the closely watched Alzheimer's drug Leqembi is off to a slow start.

Still, demand for the treatment from Biogen and its Japanese partner Eisai is rising.

That's the update the two drugmakers gave Wall Street during their latest quarterly earnings calls earlier this month.

But before we get into the numbers, here's why the treatment is such a big deal in the first place: Leqembi became the first drug found to slow the progression of Alzheimer's disease to win approval in the U.S. when the Food and Drug Administration cleared it in July. 

While the therapy isn't a cure, its rollout was a historic moment for the more than 6 million older Americans who have the hard-to-treat condition. 

Leqembi is also critical to the future of Biogen's business.

The company is pinning its hopes on the drug to drive growth as it cuts costs and sees sales plummet for its multiple sclerosis therapies, some of which face generic competition. Leqembi also marks a promising new chapter for Biogen's Alzheimer's portfolio after the polarizing 2021 approval and launch of an older drug, Aduhelm, which the company dropped last month. 

Only about 2,000 patients are currently on Leqembi, seven months after it won FDA approval, Biogen CEO Chris Viehbacher told reporters on a media call last week. That makes Biogen's target of 10,000 patients treated by the end of March look increasingly difficult to hit. 

But Viehbacher said the company had expected more of a "progressive ramp" and is seeing considerable demand for the drug. 

Biogen estimates that 3,800 patients have signed up on multiple registries to receive Leqembi as of last month, meaning they are prescribed or close to getting prescribed the drug, according to Viehbacher. He said that suggests 260 to 265 patients were being added to registries each week as of January, about a 56% increase from December. 

Morgan Stanley analysts called that registry data "encouraging" in a note last week, but said the firm is waiting to see those patients begin the therapy.

Viehbacher argues that demand isn't the problem. The real hurdle is having systems that can "accommodate this new flow of patients." 

Viehbacher is referring to the complex process patients have to go through before they receive Leqembi. To get the drug, a patient often needs to secure an appointment with a neurologist, which can already be a difficult task on its own. 

The neurologist then must run special tests to confirm that the patient is at an early stage of Alzheimer's and has a sticky toxic protein called beta-amyloid, which is a hallmark of the disease. If eligible, the patient will be prescribed Leqembi, which is administered as a one-hour infusion every two weeks.

Viehbacher also highlighted challenges related to MRI monitoring during a separate earnings call with investors. Leqembi's prescribing label says patients should get multiple MRIs during the first year of treatment to check for signs of ARIA, a side effect that causes brain swelling or bleeding and can be fatal in rare cases. 

But "when we initiate the infusion, you have to have the first MRI within the first two weeks," Viehbacher said during the call. So, some patients don't want to start the drug until they have that first MRI appointment scheduled, he said. 

"Until people get the hang of this, getting all that coordination, I think that seems to be where … one of the bottlenecks is," Viehbacher said.

Leqembi brought in $7 million in fourth-quarter revenue and $10 million in full-year sales, according to Eisai. Wall Street analysts are looking for far more this year. 

"Growth is nice - but we need to see big acceleration," Jefferies analysts wrote in a note last week. 

Notably, Biogen and Eisai are also working toward FDA approval of an injectable version of Leqembi, which showed promising initial results in a clinical trial in October. 

The injectable form would be a new and more convenient option for administering the antibody treatment to patients, which could pave the way for higher uptake. 

And increased use can't come soon enough for the drugmakers: Eli Lilly's own Alzheimer's drug, donanemab, could win FDA approval any day this quarter. 

The latest in health-care technology

Headsets are making some users nauseous. Here's what experts say can help

It's been a few weeks since Apple launched its new mixed reality headset, the Vision Pro, after months of anticipation. CEO Tim Cook told CNBC's Jim Cramer that the headset is "tomorrow's technology today," and CNBC's Todd Haselton called it the "most fun" new product he's tried in years. 

But headsets are not for everyone. Some initial users have said they experienced discomfort like headaches and motion sickness. 

For instance, in a post on the social media site X, one user said Apple's Vision Pro caused severe motion sickness, "just like all other VR." Another person experienced nausea less than 30 minutes into wearing the device, and they urged others to "proceed with caution" if they get carsick often. Even so, this person called the Vision Pro a "really incredible experience," according to an X post.

Apple released a list of tips to help users if they experience motion sickness while using the Vision Pro, like taking a break, reducing the size of their window and avoiding experiences that feature lots of movement. The company said users should talk with their doctor if their symptoms are severe and persistent, according to the site.  

Feelings of nausea and headaches are not unique to Apple's device, and users have encountered these issues with headsets from companies like Meta for years. CNBC spoke with some experts to understand why some people have trouble wearing VR, AR and mixed reality headsets. 

Dr. Joanna Jen, a neurologist at Mount Sinai Health System, said much about motion sickness is still unknown, though there are some leading theories about why it happens. She said one popular theory is that motion sickness occurs when there is an incompatibility, or mismatch, between the sensory inputs the brain and body are receiving. 

For instance, if you are flying in a headset but standing relatively still, that would be a sensory mismatch.  

Jen said everyone has a different sensitivity, and some people may adapt and build up a tolerance for the headsets with time. She said if you feel like you need to take a break, then do so, and you can always try again later.

"Moderation is key," Jen said in an interview. "Don't overdo it."

Dr. Natascha Tuznik, an infectious disease specialist and associate professor at the University of California, Davis, helps run the school's travel clinic, where motion sickness comes up often. With headsets specifically, she said users can try to alleviate symptoms by reducing the length of their sessions, ensuring that the device is fitted and set up properly, moving by transporting or warping within the headset instead of walking where applicable and looking for games that have a static reference point. 

Tuznik said there's not a great way to predict whether you'll experience motion sickness, so testing out a headset at a friend's house or a store could be worthwhile. 

"If you know how you react to non VR situations in which you can become motion sick, for example, how you react in a car or on a ride or on a boat, then it's very likely that you're going to experience a similar degree of effects when you're using VR,"  Tuznik told CNBC.

In essence, listen to your body! Take note of how you are feeling when wearing an immersive headset, and don't force it if it doesn't seem right. Unfortunately, some tech just doesn't work for everyone. The silver lining, however, is that nausea could save you $3,500 at the Apple Store.

Feel free to send any tips, suggestions, story ideas and data to Annika at annikakim.constantino@nbcuni.com and Ashley at ashley.capoot@nbcuni.com.