Merck & Co. closed out 2025 on a high note, reporting fourth-quarter revenue of $16.4 billion—a 5% increase from the prior year—and adjusted earnings per share of $2.04 that edged past analysts' consensus estimate of $2.01. But the Rahway, New Jersey-based pharmaceutical giant's cautious forward guidance sent a sobering message about the challenges ahead, as the industry's most consequential patent cliff in a decade begins to reshape the competitive landscape.

For 2026, Merck projected revenue between $65.5 billion and $67 billion, falling short of Wall Street's $67.6 billion expectation. Adjusted earnings guidance of $5.00 to $5.15 per share came in meaningfully below the $5.36 analysts had modeled, reflecting the company's candid assessment of the headwinds it faces as several blockbuster drugs lose exclusivity.

Keytruda: The $30 Billion Question

At the center of Merck's challenge stands Keytruda, the cancer immunotherapy that has become one of the best-selling drugs in pharmaceutical history. The treatment generated $8.37 billion in fourth-quarter sales alone, growing 7% year-over-year and matching analyst estimates of $8.35 billion. For the full year, Keytruda is approaching a $35 billion annual revenue run rate.

The problem is not Keytruda's present—it's its future. The drug's core U.S. patents begin expiring in 2028, opening the door for biosimilar competition that could erode revenues by 60 to 80 percent within five years, based on historical precedent with other biologic drugs that have faced generic entry.

Merck has deployed multiple strategies to extend Keytruda's commercial life, including subcutaneous formulations, combination therapies, and expanded label indications. But analysts remain skeptical that these efforts can fully offset the revenue decline once biosimilars enter the market.

"Merck is in the uncomfortable position of having built the most successful oncology franchise in history while knowing that the clock is ticking. The 2026 guidance reflects the early tremors of what's coming in 2028 and beyond."

— Jared Holz, Healthcare Strategist, Mizuho Securities

Gardasil's China Decline Continues

The quarter's most visible soft spot was Gardasil, Merck's HPV vaccine, which posted sales of $1.03 billion—a 34% decline from the same period a year ago. The precipitous drop was driven almost entirely by weakening demand in China, where local competitors have gained share and government procurement policies have shifted unfavorably for imported vaccines.

Gardasil was once Merck's second-largest revenue contributor, generating more than $8 billion annually at its peak. The China-driven decline has already lopped billions from the franchise's revenue, and management acknowledged that a full recovery in the Chinese market is unlikely in the near term.

The silver lining is that Gardasil demand outside China remains robust. In the United States and Europe, HPV vaccination rates continue to climb as public health campaigns expand coverage to broader age groups. But these mature markets cannot offset the magnitude of the China shortfall.

The Pipeline: Racing Against the Clock

Merck's ability to navigate the post-Keytruda era depends heavily on its development pipeline, which includes several late-stage assets that management has positioned as successor growth drivers. The company has invested aggressively in oncology, cardiovascular, and immunology programs, spending $30.5 billion on research and development over the past three years.

Among the most closely watched pipeline candidates:

  • Winrevair (sotatercept) for pulmonary arterial hypertension—acquired through the $11.6 billion Acceleron Pharma deal—has launched to strong early demand and is projected to reach $4 billion to $6 billion in peak sales.
  • Subcutaneous Keytruda could offer a convenience advantage that helps retain patients even after IV Keytruda faces biosimilar pressure.
  • MK-1684, a next-generation PD-1/LAG-3 bispecific antibody, is in late-stage trials with data expected in late 2026.

The $2.5 Billion Revenue Gap

Merck specifically quantified one element of its 2026 headwinds: approximately $2.5 billion in revenue at risk from generic competition for three drugs—the Januvia diabetes franchise, Bridion (a surgical anesthesia reversal agent), and Dificid (an antibiotic for C. difficile infections). These products have already begun facing generic entry, and the company expects the erosion to accelerate throughout 2026.

The Januvia loss is particularly notable because the diabetes franchise was for years one of Merck's crown jewels. At its peak, Januvia and its combination product Janumet generated more than $5 billion annually. Generic sitagliptin entered the market in late 2025, and prices have already collapsed by more than 70 percent in some channels.

Animal Health: A Bright Spot

Not everything in Merck's portfolio is under pressure. The animal health division posted nearly $1.51 billion in sales, an 8% increase year-over-year, driven by continued growth in companion animal products and livestock vaccines. The division benefits from secular trends including rising pet ownership, increasing spending per pet, and expanding veterinary care in developing markets.

Animal health also provides Merck with a degree of diversification that is unusual among large-cap pharmaceutical companies. While the division contributes less than 10% of total revenue, its stable growth profile and high margins make it a valuable strategic asset during periods of patent-driven turbulence in the human pharma business.

What Investors Should Watch

Merck's stock has gained approximately 13% over the past year, modestly underperforming the S&P 500. The company trades at roughly 12 times forward earnings—a discount to the broader pharmaceutical sector—reflecting Wall Street's ongoing concerns about the Keytruda patent cliff.

For income-oriented investors, Merck's 3.1% dividend yield provides a meaningful return floor. The company has increased its dividend for 15 consecutive years and has the free cash flow to sustain further increases, even as patent expirations pressure the top line.

The next major catalyst comes with Merck's Investor Day, scheduled for May, where management is expected to provide a comprehensive long-term strategy update. The market will be listening for more detail on how the company plans to fill the revenue gap that Keytruda's patent expiration will create—and whether the current pipeline can generate the $15 billion to $20 billion in new revenues that analysts estimate will be needed to sustain growth through the end of the decade.

For now, Merck's Q4 results demonstrate that the company remains a formidable earnings machine. But the 2026 guidance makes clear that the patent cliff is no longer a distant threat—it's here, and navigating it successfully will define Merck's next chapter.