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The concern for a Merck (MRK) shareholder is easy to state: a large share of the company's sales comes from a single drug, its cancer treatment Keytruda. The Keytruda franchise generated $8.4 billion in the second quarter of 2026, accounting for about half of Merck's $16.6 billion in revenue. Excluding currency moves, those sales grew 4% from a year earlier. Where is that growth coming from, and will it last?

Merck Expects Slower Keytruda Growth In The United States

Management answered both questions on its August 4, 2026 call. The growth came from strong uptake in earlier-stage cancers and continued demand from patients whose cancer has spread, management said. Executives also said that U.S. Keytruda growth will moderate as the drug reaches peak penetration, nearing the largest share of patients it can reach in several key uses.

A slower Keytruda would weigh on a business that already grows a little more slowly than the broader market. Merck's revenue grew 4.5% a year on average over the three years to June 2026, compared with a 6.1% average for the S&P 500 measured the same way. For now, that gap is mild.

How Big Are Merck's Newer Drugs, And What Else Is Changing?

The newer drugs Merck named each sell less than a tenth as much as Keytruda, although several are growing fast. Winrevair, a treatment for pulmonary arterial hypertension, posted $588 million in second-quarter sales, up 75% from a year earlier. Welireg, another cancer drug, grew 67% to $271 million. Management says it sees more than $70 billion of commercial opportunity from over 20 new products. That forecast represents growth well into the next decade, not a current result.

On the same call, Merck raised its 2026 revenue forecast to between $66.3 billion and $67.3 billion, or growth of 2% to 4%. Management also addressed the period after Keytruda loses exclusivity and rivals can start selling copies. Executives described the transition as "more of a hill than a cliff," anticipating a shallow dip and a fast return to growth. That dip is management's expectation, not yet a result. Separately, Merck encountered a regional patent hurdle in Europe. On October 7, a district court in the Netherlands issued a preliminary ruling in favor of Halozyme Therapeutics, restricting the marketing of subcutaneous Keytruda across eight European jurisdictions, pending potential appeal. In September, India's Cipla said its U.S. unit would license a biosimilar version of Keytruda from China's Qilu Pharmaceutical for the U.S. market.

How Far Has Merck Stock Fallen In Past Sell-Offs?

Merck shares have declined less than the S&P 500 in four of the five market shocks since 2022. In the 2023 bond-yield shock, Merck fell 6.2% against 9.5% for the index, and in the 2023 banking crisis it fell 4.3% against 6.7%. During the 2025 tariff shock, Merck stock fell 13.0% while the S&P 500 dropped 19.0%. In the 2022 inflation shock, as the Fed raised rates, Merck slipped 4.4% against a 24.0% slide for the index. The exception occurred in a 2024 sell-off, when Merck also fell 13.0% but the S&P 500 declined only 7.8%.

Those falls all took place before the stock's latest run. Merck stock returned 66.9% in the year to October 27, 2026, against 15.0% for the S&P 500. The stock now trades at 5.3 times sales, compared with 3.2 times for the index, and that price likely assumes the newer drugs will keep growing as Keytruda's U.S. growth slows.

Keytruda poses a real risk to weigh rather than an emergency, because the drug is still growing and the newer medicines are adding sales quickly. For that risk to stay worth taking, those newer drugs need to keep up that pace. Keytruda's U.S. sales growth is the measure to watch at Merck's next quarterly report, expected on October 29, 2026.

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