The Wall You Cannot See
Imagine buying a house and discovering, years after the original deed expired, that the seller had quietly filed seventeen additional property claims, each one slightly reworded, each one giving them the right to charge you rent for another few years. That is not a real-estate scenario, but it is a remarkably accurate description of what happens to the medicines millions of Americans take every day.
The practice is called patent stacking, sometimes called “evergreening,” and it has become one of the pharmaceutical industry’s most effective and controversial tools. Rather than relying on a single foundational patent to protect a drug, companies file cascading layers of secondary patents covering everything from the pill’s coating to the angle of a tablet’s bevel. By the time one protection expires, a dozen others remain, and generic manufacturers find themselves staring at a wall of legal obstacles instead of a clear path to market.
The consequences are measurable and severe. Research on the best-selling drugs in the United States has found that many accumulated dozens of patents. AbbVie’s Humira, the world’s top-selling drug for years, had more than 130 patents granted. The original compound patent on Humira expired in 2016, yet biosimilar competitors were kept out of the U.S. market until 2023. During that window, AbbVie collected tens of billions of dollars in revenue that a competitive market might have redistributed to patients and payers.
This is not accidental. It is engineered.
How the Stack Is Built
To understand patent stacking, it helps to understand what a pharmaceutical patent actually protects. A basic drug patent, often called a “composition of matter” patent, covers the active chemical compound itself. This is the crown jewel, and it typically lasts 20 years from the date of filing. Because drugs spend years in clinical trials before reaching patients, Congress created the Hatch-Waxman Act in 1984 to give companies up to five additional years of patent term restoration, plus a period of data exclusivity. The intent was to reward genuine innovation while creating a pathway for generics once those protections ran out.
What lawmakers did not fully anticipate was the ingenuity with which companies would exploit the space around a core compound. Secondary patents can cover:
- A specific salt or polymorph of the active molecule (the same drug in a slightly different crystalline form)
- A particular dosage strength or dosage interval
- A delivery mechanism, such as an extended-release capsule
- A method of use for a specific indication
- The drug’s inactive ingredients, known as excipients
- Manufacturing processes
- Pediatric studies, which earn six extra months of exclusivity under the Best Pharmaceuticals for Children Act
Each of these patents, filed at staggered intervals, can extend market exclusivity by years. When layered together, they create a thicket that generic manufacturers must either navigate, challenge at enormous legal expense, or simply avoid. Studies of top-selling drugs have found that many of the accumulated patents were filed after FDA approval and cover aspects other than the active ingredient.
AstraZeneca’s Nexium offers a textbook illustration. When the company’s blockbuster heartburn drug Prilosec (omeprazole) faced generic competition, AstraZeneca isolated one of omeprazole’s two mirror-image molecules, called esomeprazole, patented it separately, and launched it as Nexium. Clinically, the two drugs are nearly identical in most patients. Commercially, the move reset the patent clock and generated billions in additional revenue. Critics pointed to it as an example of evergreening. The company argued esomeprazole offered genuine therapeutic benefits in certain patient populations, a claim that has been debated.
The Hatch-Waxman Loophole and the 30-Month Stay
The Hatch-Waxman Act, originally celebrated as a compromise between innovation and access, inadvertently created a mechanism that patent stackers have leveraged skillfully: the 30-month stay. Under the law, when a generic manufacturer files an Abbreviated New Drug Application (ANDA) and certifies that a listed patent is invalid or will not be infringed, the brand-name company can sue for infringement. Once sued, the FDA is automatically barred from approving the generic for 30 months while litigation proceeds, unless a court rules sooner.
This provision was designed to give innovators a reasonable chance to defend legitimate patents. In practice, it gives brand-name companies an incentive to list as many patents as possible in the FDA’s Orange Book, the official registry of drug patents. Every listed patent is another potential lawsuit, another potential 30-month stay, another period of exclusive sales.
In the late 1990s and early 2000s, some companies filed infringement suits against generic manufacturers using patents so weak they seemed designed purely to trigger the stay. The Medicare Modernization Act of 2003 closed some of those specific loopholes, but the fundamental architecture remained. Companies adapted. Instead of filing frivolous suits, they simply kept filing new patents, ensuring a steady supply of legitimately listable protections.
Robin Feldman, a law professor at UC Law San Francisco who has written extensively on pharmaceutical patents, analyzed drugs that received new patents between 2005 and 2015 and found that about 78 percent were existing drugs rather than new ones.
The Human Cost at the Pharmacy Counter
The economic arithmetic of patent stacking flows directly into what patients pay. Generic drugs typically cost 80 to 85 percent less than their brand-name equivalents, according to the Association for Accessible Medicines. When generics are delayed by even a few years, the savings evaporate.
Consider insulin. Multiple manufacturers spent years adding incremental patent protections around formulations and delivery devices for insulin products, slowing biosimilar and follow-on versions. Surveys have repeatedly found that a significant share of insulin-dependent patients in the United States ration their doses due to cost. People have died as a result. While patent stacking is not the only driver of insulin pricing (pharmacy benefit managers and rebate structures complicate the picture), the layers of intellectual property protection around insulin analogs have meaningfully limited the competitive pressure that would otherwise reduce prices.
Asthma inhalers offer another painful example. Several leading inhaled corticosteroid and bronchodilator products have accumulated secondary patents on delivery devices, propellants, and dose counters that extend market exclusivity well beyond the active molecules’ original protection. Inhalers sold in the United States cost far more than the same products in many European countries.
At a systemic level, the Association for Accessible Medicines estimates that generic drugs saved the U.S. healthcare system about $1.67 trillion between 2007 and 2016. Each additional year of delayed generic entry erodes a portion of those potential savings.
The burden falls hardest on patients with chronic conditions who take brand-name drugs for years, on insurers and employers whose premiums rise to cover the costs, and on government programs like Medicare and Medicaid that spend hundreds of billions annually on outpatient drugs.
The Industry’s Defense and the Counterargument
The pharmaceutical industry pushes back on evergreening critiques with arguments that deserve a fair hearing, because some carry genuine weight.
First, companies argue that secondary innovations are real innovations. An extended-release formulation is not trivial engineering. It can improve patient adherence, reduce side effects, and represent years of additional research investment. A delivery device designed for patients with limited dexterity, say, an arthritis-friendly inhaler, adds genuine value. Dismissing all secondary patents as rent-seeking, the argument goes, misunderstands how drug development actually works.
Second, the industry notes that developing a successful drug requires not just the successful molecule but enormous resources spent on failures. The PhRMA trade group has long cited figures suggesting it costs more than $2 billion to bring a single new drug to market, accounting for the cost of failed compounds. Secondary patent revenue, in this view, helps fund the next generation of medicines. Whether those figures are accurate is genuinely contested: a 2017 study in JAMA Internal Medicine, by researchers at Oregon Health and Science University and Memorial Sloan Kettering Cancer Center, examined 10 cancer drugs and found a median research and development cost of approximately $648 million per drug, far below the widely publicized industry estimates.
Third, companies point out that patent challenges are legally available. Any generic manufacturer can file a Paragraph IV certification challenging a patent’s validity, and courts regularly invalidate weak patents. The Inter Partes Review process at the U.S. Patent and Trademark Office, established by the America Invents Act of 2011, offers a faster, cheaper route to challenge weak patents. And sometimes it works. Humira biosimilars eventually entered the U.S. market in 2023, and Stelara (ustekinumab) faced biosimilar competition in early 2025 after a patent settlement.
These arguments, however, largely describe a system working at the margins. The broader pattern, documented repeatedly by researchers independent of either industry or advocacy groups, shows that secondary patent accumulation in pharmaceuticals is both deliberate and disproportionate compared to other technology sectors.
Reform Efforts and the Road Ahead
Policymakers have taken notice, though legislative progress has been uneven.
The Inflation Reduction Act of 2022 introduced Medicare drug price negotiation for a limited set of high-cost medicines, targeting some of the most expensive brand-name drugs whose prices have been propped up by long periods of exclusivity. The law does not directly address patent stacking, but by capping what Medicare pays for negotiated drugs, it reduces the financial reward for accumulating patents purely to extend monopoly pricing.
The Federal Trade Commission has also taken aim at the system. In 2023, under then-chair Lina Khan, it issued a policy statement on improper patent listings in the Orange Book and began sending warning letters to brand-name drugmakers, and it renewed those challenges in 2025. It has long scrutinized “pay-for-delay” settlements, in which brand-name companies essentially pay generic manufacturers to stay out of the market. The Supreme Court ruled in FTC v. Actavis in 2013 that such payments can violate antitrust law, but enforcement has been inconsistent.
On the patent examination side, reform advocates have pushed the U.S. Patent and Trademark Office to apply higher standards to secondary pharmaceutical patents, particularly those covering minor formulation changes. The office faces a structural challenge: examiners have limited time to review each patent application, across all technical fields. Pharmaceutical patent applications, which can run hundreds of pages, are not always well-served by that constraint.
Internationally, some countries have moved further. India’s patent law explicitly prohibits the granting of new patents for new forms of known substances unless they show significantly enhanced efficacy, a provision upheld by India’s Supreme Court in the Novartis v. Union of India case decided in 2013. Canada and Australia, by contrast, run their own “patent linkage” systems, which connect generic approval to patent status in different ways than the U.S. system does.
In the United States, the PREVAIL Act, proposed in Congress, would reform Inter Partes Review by requiring petitioners to show they have a real stake in a patent’s validity before initiating a challenge, a provision that critics argue would actually make it harder to knock out weak pharmaceutical patents. The legislative debate remains unsettled.
What is clear, looking at the trajectory of pharmaceutical patent litigation and pricing data, is that the current system imposes enormous and measurable costs that fall most heavily on the sick and the uninsured. The patents at the center of these disputes are not secret. They are published, listed, and publicly available to anyone with patience and a legal background. What they are not is transparent to the patients who pay the price every time they present a prescription.
A System Ripe for Reckoning
Patent stacking did not emerge from malice. It emerged from incentive structures that reward legal creativity as richly as scientific creativity, and from a regulatory framework designed for a simpler era of pharmaceutical development. Companies that exploit these structures are, in a narrow sense, doing exactly what rational actors in a poorly designed system are expected to do.
But the consequences of that rational behavior are increasingly difficult to defend on public health grounds. When a drug whose core chemistry has been in the public domain for years still commands monopoly prices because of patents on its bottle’s child-resistant cap, something has gone wrong. When biosimilars that could reduce patient costs are delayed for years by settlements and secondary patents, the innovation justification for those protections becomes very thin.
The conversation about drug pricing in the United States has, for years, focused on pharmacy benefit managers, insurance structures, and hospital markups. Those are real problems. But beneath all of them sits the patent wall, brick by brick, form by form, filing by filing, giving the original manufacturer the legal right to be the only seller in a market where competition would otherwise drive prices down.
Reforming that system requires simultaneously strengthening patent examination, narrowing the types of pharmaceutical innovations eligible for secondary protection, accelerating generic entry timelines, and reducing the financial incentives for pay-for-delay settlements. None of those changes is technically complicated. All of them are politically difficult, because the industry that benefits from the current system is among the most effective lobbying forces in Washington.
The drugs in question are, in many cases, genuine miracles of modern medicine. The patients who depend on them deserve both continued innovation and prices that do not force them to choose between a prescription and a meal. Those two goals are not mutually exclusive. They only look that way when the patent wall is left standing.