Over time, Biogen (BIIB 0.78%) created a true success story. The biotech company built up an impressive portfolio of multiple sclerosis (MS) drugs and generated billions of dollars in earnings. The share price followed, climbing more than 20,000% from its initial public offering to its peak about eight years ago.

But recent years haven't been as easy. Competition for Biogen's MS blockbuster Tecfidera, and for other key products, entered the market. That's resulted in sales declines -- and questions about where growth will come from down the road.

Now, though, Biogen may have found that new growth source. And a regulatory decision in July may make things official. Should you buy Biogen before the July news? Let's find out.

A blockbuster portfolio

First, let's take a quick look at Biogen's past and current situations. Tecfidera and a blockbuster portfolio of MS drugs led the company's revenue and profit higher over time. Rituxan, used to treat certain autoimmune conditions and types of cancer, also made significant contributions. Loss of exclusivity has been weighing on those products, though, and will continue to hurt market share.

BIIB Net Income (Annual) Chart

BIIB Net Income (Annual) data by YCharts.

To ignite a new phase of major growth, Biogen set its sights on an area with great need: Alzheimer's disease. The company began developing treatment candidates with partner Eisai (ESAI.Y -0.70%). The idea was to target the accumulation of beta amyloid thought to be interrupting cell function in the brain. The companies' first Alzheimer's product, launched in 2021, met with controversy.

Biogen had originally halted development of the drug, Aduhelm, after disappointing trial results -- then revived the program after reviewing data from a subgroup of patients.

The scientific and medical communities didn't back Aduhelm. And the Centers for Medicare and Medicaid Services said it only would cover the drug -- or other monoclonal antibodies targeting beta amyloid -- for those participating in clinical trials.

Biogen and Eisai gave up on Aduhelm. But their interest in conquering Alzheimer's isn't over -- in fact, it's just getting started. They recently won accelerated approval for Leqembi, another beta-amyloid-targeting treatment. Unlike Aduhelm's, Leqembi's clinical trial results have been more straightforward: They showed 37% improvement in patients' daily life activities. So the medical community may be more supportive of Leqembi than it was of Aduhelm.

Reviewing Leqembi for full approval

The U.S. Food and Drug Administration is now reviewing Leqembi for full approval, and it's set to make a decision by July 6. Full approval is important because it should lead to improved coverage -- making it easier for more patients to afford the drug. Medicare said that if Leqembi wins traditional approval, it will broaden its coverage of the treatment.

Meanwhile, in a positive move for Biogen and Eisai, the U.S. Veterans Health Administration has agreed to cover Leqembi.

Let's consider how Leqembi may impact Biogen's revenue. Biogen and Eisai co-commercialize and co-promote the product, so they'll share in expenses and profit. Eisai forecasts $7.3 billion in worldwide sales for the drug by 2030, according to The Pharma Letter. Today, Biogen's full-year revenue totals about $10 billion -- so even after sharing its profits with Eisai, Leqembi could become an important revenue source.

But gains won't happen overnight. In fact, Biogen expects Leqembi to weigh on earnings this year. That's because the costs to commercialize the drug will surpass revenue it brings in.

It's also important to remember that getting Leqembi to patients isn't a quick and easy process. A couple of challenges are involved. Patients first must get a positron emission tomography (PET) scan or a lumbar puncture to diagnose disease. Then, treatment is given by infusion at an infusion center; here, a potential roadblock could be capacity, as doctors need infusion centers to administer many types of drugs for various illnesses. All of this could delay Leqembi treatments -- and, as a result, product revenue.

What does all of this mean for investors?

There are definitely reasons to be optimistic about Leqembi. But, right now, there are a lot of moving parts -- drug uptake, potential reimbursement levels -- to the story. And only when these moving parts settle will we have a clear idea of how much revenue Leqembi will bring to Biogen over time.

For this reason, and considering declines in Biogen's older drugs, it's also hard to use traditional valuation methods to evaluate Biogen shares.

So should you buy Biogen before the July decision? If you're willing to accept some near-term risk and a potentially long path to growth, you might consider it. The stock has dropped about 40% from its record high back in 2015.

Biogen is likely to become a high-growth biotech again, via Leqembi and/or other products. But investors may have to be patient as the company finds just the right formula to get there.