Biogen (BIIB -0.11%) is one of the biotech-industry's giants and success stories. At its peak in 2015, the company's market value topped $100 billion. And its annual revenue reached more than $14 billion at its highest, thanks to its blockbuster multiple sclerosis drugs.

But Biogen's portfolio is aging -- and that means generic competition has started to weigh on the revenue of its biggest products. At the same time, the company is pushing new candidates through the pipeline to drive future growth.

Good news about one of those drug candidates has driven the shares higher this year. Biogen stock has climbed 17%. Can it continue? Let's check out one green flag and one red flag.

Green flag: The potential of lecanemab

Biogen's 2022 share gains don't reflect its first months of the year. The company gave up on its controversial Alzheimer's drug, Aduhelm. The medical community wasn't convinced of Aduhelm's safety and efficacy, and Medicare refused to reimburse for its use.

But Biogen may still score a win in Alzheimer's treatment. The candidate it's developing with partner Eisai recently met its primary endpoint and all major secondary endpoints in a phase 3 trial. Lecanemab is meant to treat mild cognitive impairment due to Alzheimer's disease.

The companies said lecanemab delivered "highly statistically significant results." The candidate reduced clinical decline in patients, compared to those taking a placebo over a period of 18 months.

Eisai now aims to apply for approval in the U.S. by the end of March. If all goes well, Biogen could have another Alzheimer's product -- and one with solid efficacy and safety data -- on the market in the not-too-distant future.

Analysts have forecast as much as $14 billion in annual revenue for lecanemab. Biogen shares profits 50-50 with Eisai. Even shared, this kind of revenue is huge for Biogen. Last year, the company reported total revenue of about $10 billion.

Red flag: Declining portfolio revenues

Lecanemab news has been great so far, but Biogen is depending on that one potential product right now. That's because, as mentioned above, its biggest revenue drivers are facing generic competition. Without lecanemab, Biogen's near-term revenue drivers seem very limited.

In the most recent earnings period, Biogen said it expects continued erosion of sales of both multiple sclerosis drug Tecfidera and immunology drug Rituxan.

In the case of Tecfidera, Biogen commercialized a new drug a few years ago that was meant to take over from its big blockbuster. But so far, the drug -- Vumerity -- hasn't shown it can compensate for lost Tecfidera sales.

For example, in the most recent quarter, Vumerity sales totaled about $137 million. And last year, Vumerity's full-year sales of about $410 million were far from blockbuster territory.

Of course, Biogen has nearly 30 candidates in the pipeline, with a specialty in neuroscience. Many candidates are in late-stage development, too, which is great news. But until one or more make it through to commercialization, Biogen faces a future of declining sales.

What does this mean for investors?

Today, Biogen's current portfolio of products isn't set to drive future growth. But the pipeline looks promising -- and lecanemab could be a game changer for Alzheimer's patients and Biogen.

Should you invest in Biogen today? It depends on your comfort with risk. Biogen has reached a critical point with the upcoming lecanemab regulatory submission. A lot is riding on the approval and successful launch of the potential product, and any difficulty could weigh on the shares. The most cautious investors may want to wait before buying Biogen right now.

But if you're comfortable with a bit of risk, today could be a great time to bet on this biotech giant. Biogen may be on its way to a whole new era of growth, and that could lead to share gains over time.