AbbVie (ABBV 0.01%) and Pfizer (PFE 0.12%) are makers of two of the world's top-selling pharmaceutical products. AbbVie's immunology drug Humira brought in peak sales of more than $21 billion, while Pfizer's coronavirus vaccine generated peak sales of $37 billion -- in 2022, in both cases. With results like those, you might expect their stocks to still be soaring, but they aren't.

Instead, AbbVie and Pfizer shares have dropped by double-digit percentages this year, and for reasons that are understandable. The companies have something in common. Sales of both of their top-selling products are on the decline in a big way. Humira has lost exclusivity, and demand for Pfizer's coronavirus vaccine is declining now that we have exited the health-crisis stage of the pandemic. But does this mean you should follow the crowd and sell shares of these companies?

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AbbVie and the world's top-selling drug

We'll start with AbbVie. The company has a full portfolio of products in areas including immunology, neuroscience, and aesthetics, but Humira has long stood out because it's useful for patients across a wide variety of indications -- helping treat symptoms of illnesses ranging from rheumatoid arthritis to Crohn's disease. This has led to increasing sales, which peaked at a level that consistently made it the world's top-selling drug in recent years.

Of course, drugs eventually lose patent protection, so Humira's growth couldn't continue forever. However, AbbVie has prepared for this moment. The company has successfully brought a pair of newer immunology drugs -- Rinvoq and Skyrizi -- through the pipeline, and both drugs are now approved for various indications that overlap with Humira's. The goal is for the two of them, together, to surpass Humira's peak sales by 2027. And it looks like that may happen.

In the most recent quarter, Rinvoq and Skyrizi grew sales by double-digit percentages, and they're on track to deliver more than $11 billion in revenue this year. AbbVie has won five approvals for Rinvoq and three for Skyrizi -- and more may be on the way. The idea is these drugs will cover all seven of the areas Humira treats.

Meanwhile, AbbVie also can count on other blockbusters for growth, among them bipolar disorder drug Vraylar and migraine treatment Botox.

Of course, Humira's sales decline -- a 36% year-over-year drop in the latest quarter to $3.5 billion -- is weighing on earnings. And it will take time for Rinvoq and Skyrizi to compensate for those declines and grow. But considering their performances so far and AbbVie's full pipeline, it should be worth the wait. And that's why I wouldn't sell shares of AbbVie right now.

Pfizer and the world's top-selling pharma product

Pfizer's coronavirus vaccine, Comirnaty, was last year's top-selling pharmaceutical product, with its revenue even beating that of Humira. But Comirnaty's story isn't as long as that of AbbVie's biggest blockbuster.

Comirnaty, launched in late 2020 during a pandemic situation, quickly rose to the top, but now, with so many people worldwide having received at least their initial inoculations against COVID, demand for coronavirus vaccines and boosters is on the decline.

Based on the level of demand during this fall's vaccination season, Pfizer recently revised its annual guidance for Comirnaty revenue downward by $2 billion.

However, this hardly means all is lost for Comirnaty, nor for Pfizer. Comirnaty will likely continue generating significant recurrent revenues -- though not as high as during the peak years of the crisis -- for Pfizer every fall. A certain percentage of the population will continue to want annual coronavirus boosters.

Pfizer faces an additional challenge: It's set to lose exclusivity on certain blockbusters this decade, and those predictable events have led management to anticipate $17 billion in lost revenues from 2025 through 2030. But the pharma giant also has some good news for investors. It's in the middle of an 18-month stretch that features one of its busiest times for new product launches ever. It set a goal to bring 19 new products or indications to market by the end of that period, and it's most of the way there, with 13 launched. These products and Pfizer's business deals could help it reach $84 billion in revenue in 2030. That wouldn't be as high as the $100 billion in sales it reached last year thanks to Comirnaty and the COVID-19 antiviral Paxlovid. But it would be more than 60% higher than its pre-COVID revenue levels.

Pfizer and AbbVie actually have something else in common beyond the fact that their biggest sellers are on the decline. Both have paved new paths toward growth -- and those paths look promising. So, like AbbVie, Pfizer is more of a buy than a sell today.