Merck Stock Surges More Than 50% in Six Months: Should Investors Buy, Hold, or Lock In Gains?

Merck & Co. has become one of the standout names in big pharma after its shares climbed roughly 51.4% over the past six months, according to a Zacks commentary published on March 27, 2026. The rally reflects stronger investor confidence in the company’s long-term growth story, especially as Merck works to prepare for the eventual loss of exclusivity for its blockbuster cancer drug, Keytruda. At the same time, the stock’s sharp rise has sparked an important question for investors: after such a strong move, is Merck still a buy, a stock to hold patiently, or a candidate for profit-taking?

Why Merck Stock Has Been Climbing So Fast

The recent jump in Merck shares has not happened by accident. Investors have been rewarding the company for showing a clearer strategy for life after Keytruda, which has been the single biggest force behind Merck’s pharmaceutical growth in recent years. Keytruda remains one of the world’s best-selling medicines and generated about $31.7 billion in 2025 sales, up 7% year over year. That performance matters because the drug represents around 55% of Merck’s pharmaceutical revenue, meaning confidence in the company often rises or falls with confidence in Keytruda.

For a long time, one of the market’s biggest worries was that Merck depended too heavily on this one treatment. Investors feared that once Keytruda lost patent protection, Merck would struggle to replace such a huge revenue stream. Recently, however, sentiment has improved because the company has begun to look less like a one-drug story and more like a diversified pharmaceutical group with several possible future growth engines.

That shift in perception is a major reason the stock has surged. Markets tend to reward companies when the future looks more visible, and Merck has given investors more reasons to believe that revenue growth may continue even after the Keytruda era begins to fade. This does not remove all risk, but it does explain why the stock has outperformed many peers and drawn renewed attention from long-term investors.

Keytruda Remains Merck’s Biggest Strength

A Blockbuster That Still Has Room to Run

Keytruda is still the centerpiece of the Merck investment case. The immunotherapy medicine has become a standard treatment across many cancer types, and its continued expansion into earlier-stage disease settings has helped keep demand strong. Zacks noted that sales have been supported by rapid uptake in early-stage indications as well as solid momentum in metastatic cancer uses. Merck expects this strength to continue until the drug faces loss of exclusivity around 2028.

That matters because investors are not only buying Merck for what it is today; they are also buying time. If Keytruda can continue growing or remain resilient for the next few years, Merck gets more breathing room to develop new products, complete acquisitions, and scale fresh revenue sources before the patent cliff arrives. In other words, the stock’s recent rally reflects confidence not just in current earnings, but in the runway that current earnings provide.

New Strategies to Extend the Franchise

Merck is also trying to make the Keytruda franchise more durable through new combinations and improved formulations. According to the Zacks report, the company is advancing innovative immuno-oncology combinations involving LAG3 and CTLA-4 inhibitors. It is also developing the personalized cancer vaccine intismeran autogene, also known as V940 or mRNA-4157, with Moderna in pivotal phase III studies for earlier-stage and adjuvant non-small cell lung cancer and melanoma. In addition, the U.S. FDA approved the subcutaneous version of the drug, Keytruda Qlex, in September 2025, which offers much faster administration than the traditional intravenous form.

These efforts show that Merck is not simply waiting for exclusivity to expire. It is actively trying to squeeze more value, convenience, and clinical usefulness out of one of the most important oncology brands in the world. For shareholders, that improves the odds that Keytruda can remain a meaningful earnings engine for as long as possible.

Merck’s Pipeline Is Starting to Matter More

Late-Stage Programs Have Expanded Significantly

One of the strongest reasons behind the market’s changing view of Merck is pipeline depth. Zacks reported that Merck’s phase III pipeline has nearly tripled since 2021, helped by both internal research progress and business development. That is a major improvement for a company that needs to prove it can build a durable second act beyond Keytruda.

Investors usually become more confident when a drugmaker has multiple late-stage programs rather than relying on one or two headline assets. A broad pipeline reduces the damage if one program disappoints. It also increases the chance that several products can become meaningful contributors over time. Merck’s recent stock run suggests the market is beginning to see this broader portfolio as real, not just theoretical.

Important New Products Are Gaining Traction

Among the newer growth drivers, Zacks highlighted Capvaxive, a 21-valent pneumococcal conjugate vaccine, and Winrevair, a treatment for pulmonary arterial hypertension. Both have launched strongly and are viewed as products with significant long-term revenue potential. Merck also received U.S. approval in June 2025 for Enflonsia, its RSV antibody clesrovimab, while a fixed-dose HIV combination of doravirine and islatravir is under FDA review, with a decision expected in April 2026.

The company also has other notable late-stage or advancing assets, including oral PCSK9 inhibitor enlicitide decanoate, ulcerative colitis candidate tulisokibart, and partnered antibody-drug conjugates with Daiichi Sankyo. None of these alone may replace Keytruda dollar for dollar in the near term, but collectively they support the argument that Merck is assembling a multi-engine growth model.

Acquisitions Have Helped Change the Story

Merck has been especially active on the dealmaking front, and that has played a major role in the stock’s rise. The company has clearly recognized that internal development alone may not be enough to offset the looming Keytruda patent cliff. Instead, it has used acquisitions to add assets in respiratory disease, infectious disease, and oncology.

According to the Zacks analysis, Merck’s 2025 acquisition of Verona added Ohtuvayre, a first-in-class maintenance treatment for chronic obstructive pulmonary disease with multibillion-dollar commercial potential. In January 2026, Merck acquired Cidara Therapeutics, bringing in MK-1406, a long-acting antiviral candidate being studied for seasonal influenza prevention in high-risk people. Then, in the same week as the Zacks article, Merck announced an agreement to acquire Terns Pharmaceuticals for $6.7 billion, adding TERN-701, an early-stage treatment candidate for certain chronic myeloid leukemia patients.

Investors often like this kind of proactive behavior. It shows management is not ignoring the future and is willing to spend now to support the next decade of growth. Of course, acquisitions carry execution risk and may pressure earnings in the short term. Still, the market appears to believe these moves have meaningfully improved Merck’s long-range outlook.

Animal Health Is an Underrated Business for Merck

Another point supporting the bullish case is Merck’s animal health segment. While it does not attract the same headlines as oncology, Zacks noted that this business remains an important contributor to top-line growth and could more than double sales by the mid-2030s. That matters because non-human health revenue helps diversify the business and reduces dependence on one category or one blockbuster medicine.

For long-term investors, businesses like animal health can be especially attractive because they often provide steadier demand patterns than some pharmaceutical segments. They may not move the stock in dramatic fashion overnight, but they can improve the quality and resilience of overall corporate earnings. Merck’s valuation has likely benefited from this diversification story as well.

The Biggest Risks Investors Should Not Ignore

Keytruda Patent Expiration Still Looms

Even with all the optimism, the biggest risk has not gone away. Keytruda’s core U.S. patent is expected to expire around 2028, and biosimilar competition is expected to arrive around 2028 to 2029. Once lower-cost alternatives appear, sales could come under heavy pressure. Since Keytruda is such a large portion of Merck’s pharmaceutical business, any decline would be felt sharply.

This is why some investors may hesitate to chase the stock after a 50%-plus move. A company can look strong today and still face a very difficult transition a few years later. The entire Merck debate comes down to whether its new products and pipeline advances will be enough to bridge that gap. The answer is becoming more encouraging, but it is not fully settled.

Competitive Threats in Oncology Are Rising

Zacks also pointed to a newer concern: competitive pressure from dual PD-1/VEGF inhibitors. These therapies aim to block both the PD-1 and VEGF pathways at the same time and could challenge single-target treatments like Keytruda. Even before patent expiry, stronger competition could affect growth expectations or force Merck to defend market share more aggressively.

That does not mean Keytruda is about to be displaced overnight. However, investors in pharmaceutical stocks know that market leadership in oncology can shift quickly when better combinations, better convenience, or better outcomes emerge. Merck still has enormous scale, but leadership positions must be defended continually.

Gardasil Weakness Has Been a Real Drag

While Keytruda has remained a major strength, Merck’s second-largest product, Gardasil, has gone through a difficult period. Zacks reported that Gardasil sales fell 3% in 2024 and then dropped 39% in 2025, mainly because of weak demand in China. The report also noted softer demand in Japan and suggested that Gardasil sales are not expected to improve in 2026.

This is an important issue because investors would ideally like Merck to have several large franchises growing at the same time. Instead, one of its key vaccine businesses has become a headwind. Sales of other vaccines such as ProQuad, M-M-R II, Varivax, Rotateq, and Pneumovax 23 also declined in 2025, according to the same analysis.

In simple terms, Merck’s pipeline and acquisitions may be improving the future, but parts of the current business are still under pressure. That mixed picture helps explain why the stock may be attractive for patient investors yet less compelling for traders looking for easy upside after a rapid surge.

Generic Pressure Could Hurt Near-Term Revenue

Another challenge lies in older products facing generic competition. Zacks said Merck is seeing falling demand for diabetes drugs Januvia and Janumet, as well as generic erosion affecting products such as Isentress, Bridion in the European Union, and Dificid in the United States. The report also said Bridion is expected to lose U.S. patent exclusivity in July 2026.

Perhaps more importantly, Zacks wrote that Merck expects generic competition involving Januvia, Janumet, Bridion, and Dificid to reduce 2026 revenue by approximately $2.5 billion. That is a meaningful drag and a reminder that new growth drivers are not arriving in a vacuum. They are arriving while older products are weakening.

For investors, this creates a split timeline. The long-term picture may be improving, but the next few quarters could still look uneven as headwinds from generics, vaccines, and deal-related costs move through the income statement. Stocks sometimes pause after a big run when near-term fundamentals stop accelerating, even if the long-term story stays intact.

Valuation and Estimate Trends Suggest Caution

The stock’s performance has been impressive. Zacks reported that Merck shares rose 33.2% over the past year, better than the broader industry, sector, and the S&P 500. The shares have also been trading above their 50-day and 200-day moving averages since early November, a sign of technical strength.

Still, the valuation picture is not as cheap as it used to be. Based on the Zacks analysis, Merck trades at about 18.27 times forward earnings, slightly above the industry average of 16.99 and also above its own five-year average multiple of 12.61. In other words, investors are already paying a higher price for the company’s improved outlook.

Analyst estimates have also moved lower. Zacks said the consensus earnings estimate for 2026 fell from $6.55 per share to $5.47 over the past 60 days, while the 2027 estimate declined from $10.07 to $9.89. Falling earnings estimates combined with a rising stock price can make valuation look stretched in the short run. That does not necessarily mean the stock is overvalued beyond repair, but it does support a more selective approach after such a strong rally.

So, Should Investors Buy, Hold, or Take Profits?

The Case for Holding

The strongest conclusion from the recent analysis is that Merck still looks like a reasonable holding for long-term investors. The company owns one of the most valuable oncology franchises in the world, its newer products are gaining traction, its late-stage pipeline has expanded significantly, and management has used acquisitions to build future growth platforms. Zacks also said Merck sees more than $70 billion in potential non-risk-adjusted commercial opportunity from its current pipeline by the mid-2030s, more than double the peak consensus sales estimate for Keytruda in 2028.

That kind of statement helps explain why many shareholders may prefer patience over panic. The company’s future is far from risk-free, but the long-range blueprint is much clearer today than it was a year ago. Investors who believe management can execute may decide that staying invested is the smarter move.

The Case for Taking Some Profits

On the other hand, short-term investors may reasonably decide to trim positions after a 50%-plus six-month gain. The stock already reflects a lot of optimism. Near-term challenges remain, including weak Gardasil demand in China, generic competition, estimate cuts, and the possibility that acquisition costs weigh on results before the benefits fully show up. Zacks ultimately described Merck as a Zacks Rank #3, or Hold, which suggests balanced expectations rather than a clear near-term buy signal.

For momentum investors, that matters. When a stock has already made a dramatic move and analysts still see limited short-term growth, some investors will prefer to protect gains and wait for a better re-entry point. That is not a bearish verdict on Merck’s business. It is simply a recognition that stock prices can get ahead of fundamentals for periods of time.

The Case for Buying

A fresh buy case still exists, but it is more suitable for investors with a long time horizon and tolerance for volatility. Someone buying Merck today is effectively betting that the company’s combination of Keytruda durability, pipeline breadth, and strategic acquisitions will outweigh the future patent cliff. That may prove correct, but it likely requires patience. This is less a “quick upside” story now and more a “multi-year execution” story.

Bottom Line

Merck’s massive six-month rally reflects a real change in how investors view the company. Instead of focusing only on the danger of losing Keytruda exclusivity, the market has started to pay more attention to Merck’s expanding late-stage pipeline, promising launches such as Winrevair and Capvaxive, support from animal health, and strategic acquisitions including Verona, Cidara, and Terns. That is the heart of the bullish argument, and it is a serious one.

Still, investors should stay grounded. Keytruda dependence remains high, Gardasil is under pressure, generic erosion is real, and estimates have softened. After such a strong run, Merck may no longer offer the same easy upside it did several months ago. A sensible reading of the situation is that long-term holders may want to stay the course, while shorter-term investors may consider taking partial profits after the rally. That balanced stance lines up closely with the overall message in the March 27, 2026 Zacks commentary.

Source context: This rewritten article is based on reporting and analysis summarized from Zacks commentary titled “Merck Stock Soars 50% in 6 Months: Buy, Hold, or Take Profits?”, along with search-result excerpts referencing the same March 27, 2026 report. For readers who follow pharmaceutical equities, the Merck story remains one of the most important case studies in how markets price both blockbuster success and patent-cliff risk at the same time.

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