Biogen (BIIB 0.38%) has been a huge winner for investors who owned the stock in anticipation of U.S. Food and Drug Administration (FDA) approval of its Alzheimer's disease drug, Aduhelm. In this Motley Fool Live video recorded on June 9, Motley Fool contributors Keith Speights and Brian Orelli discuss one reason why Biogen isn't a buy after its key FDA victory.

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Keith Speights: Let's segue then to the obvious question here. The question that a lot of investors are asking now is, "Should I buy Biogen stock after this big FDA approval for its Alzheimer's disease drug?" Brian, what should investors be factoring in to their decision-making process right now? What's your take on whether or not Biogen is a buy right now?

Brian Orelli: From a valuation standpoint, it's easy to look at historical data and see where they've been trading. I tend to look at price-to-sales just because drug companies, if you use the earnings, it's a little more difficult because they have a lot of one-time events and acquisitions and things like that and licensing deals that throw off their GAAP earnings. Then if you're looking at historical earnings, they usually don't back those out, so I use price-to-sales as a metric.

Biogen is trading at least after Monday's, after Monday's jump, it was at 4.9 price-to-sales ratio. Last time it traded at that level was in the 2018, 2019 time frame, and at that point, revenue was hovering around 10% growth.

To justify this price-to-sales ratio, you have to expect that sales are going to grow by 10% per year. Revenue was $13.45 billion in 2020. You need to get to $14.8 billion to get to that 10% growth. Guidance for next year is for it to drop to $10.45 billion to $10.75 billion, and they had already factored in modest revenue from Aduhelm in 2021.

At the high end, Biogen needs $4 billion in sales to justify the current valuation. Four billion would be completely reasonable for an Alzheimer's disease drug, and $10 billion would probably be reasonable, but that would be a drug that actually helps patients. As we've said, there's not enough data to know whether Aduhelm actually helps patients.

Medicare, they've got the drug approved so that's good, but they need to get the doctors to prescribe it, and they need to get insurers to cover it. Medicare will probably cover it, but it's an infused product and that means it's covered under Medicare Part B, B as in boy, not D as in drugs. That comes with a 20% copay after reaching the deductible. We're talking about $10,000 for the patients they're going to have to pay on the drug. That's going to really limit sales.

If you want to look at it at a different way, if you assume the valuation already factors in the fall to around $10.5 billion and then you are looking for 10% growth from there, now you only need $1 billion or so in sales to justify that growth. Maybe that seems a little more doable, but then that assumes that the revenue from the multiple sclerosis drugs, Tecfidera and Rituxan, that are causing the drop this year, and that you have to assume that's going to just stop.

I don't think that's going to stop, so that means either they're going to generate more than $1 billion to justify 10% growth from 2021-2022. Very long story short, I have a hard time seeing the value of investing at this level.