Thousands of Americans will soon gather to celebrate April 20 – or “4/20” – the most important day of the year for cannabis enthusiasts.

But this year, a cloud of uncertainty will hang over these celebrations. After years of success, the movement to legalize recreational and medical cannabis has stalled.

It’s a moment unlike any that I have seen in the 12 years that I’ve been researching cannabis legalization as part of my broader interest in U.S. drug policy.

Not so long ago, the movement had so much momentum that nationwide cannabis legalization felt virtually inevitable. That momentum is now gone.

The strategy to legalize cannabis through ballot initiatives is no longer working. The coalition of supporters that made this strategy work has frayed, and new research is raising concerns about the health impact of regular cannabis use. All of this constitutes the most significant challenge to the movement since it went mainstream in the 21st century.

Years of success

As a social movement, cannabis legalization has been extremely successful. Since 2012, 24 states and Washington have legalized recreational cannabis use. Forty-nine states and Washington have legalized medical cannabis use, though programs vary from state to state.

While cannabis remains illegal at the federal level, changes have happened there, too.

The 2018 Farm Bill, for instance, legalized hemp, a non-psychoactive derivative of the cannabis plant used to make textiles, rope and other consumer goods. While it wasn’t lawmakers’ intent, entrepreneurs figured out how to make products from hemp that contain enough of the chemical compound tetrahydrocannabinol, or THC, to be psychoactive. This fueled growth of the hemp market, which in 2023 was valued at US$1.63 billion.

Additionally, the Biden administration in 2024 began the process of rescheduling cannabis under the Controlled Substances Act. It’s a course that has continued under the second Trump administration.

The scheduling system classifies substances based on accepted medical use and potential for abuse. Federal rescheduling would not legalize cannabis, but it would move it from the most restrictive Schedule I – which includes substances like heroin and LSD – to Schedule III, with substances like anabolic steroids, ketamine and codeine. It would recognize cannabis as having medical use.

Challenges emerge

With rescheduling still underway, it may seem odd to say that the legalization movement has stalled. But a closer look reveals significant challenges.

The biggest challenge can be found at the ballot box. The 2024 election was the legalization movement’s worst showing in years.

All three recreational legalization ballot measures failed. Only Nebraska’s medical legalization measures passed, but it has yet to be fully implemented due to ongoing political and legal challenges.

Then there’s the 2025 tax and spending package approved by Congress. When its new provisions go into effect later this year, they will dramatically alter the hemp market.

Many hemp products currently on shelves, like THC-infused beverages and gummies, will become illegal. Many businesses currently selling these products will be forced to close.

Some of this is already happening, as states like Tennessee and Iowa rush to pass restrictions on hemp products.

For instance, the dispensary closest to my university in Iowa has just closed. Once a growing business that employed 30 people, it was forced to shut down after new state laws significantly limited what they could sell. This crackdown on the hemp market is particularly significant in states like Iowa that have no legal market for recreational marijuana use and only a limited medical marijuana market.

No single reason for current slump

Several factors are driving these changes.

One is politics. While the vast majority of Americans support marijuana legalization, the approval is much higher among Democrats and independents than it is among Republicans.

Of the 26 states where recreational marijuana has not been legalized, 20 of them have state governments that are under total Republican control. Another four have Republican-controlled legislatures. Pennsylvania’s legislature is split between Republicans and Democrats. Only Hawaii has a Democrat-controlled state government that has not legalized recreational cannabis.

Then there is the health issue. A growing body of evidence is raising concerns about the negative impact of regular cannabis use that includes the risk of cannabis addiction, psychosis, anxiety and depression.

Researchers are also questioning cannabis’ efficacy as medicine. Several recent reviews have concluded that there is insufficient scientific evidence to support the therapeutic use of cannabis for most of the conditions for which it is consumed, such as insomnia and acute pain. A review of cannabis’s use for treating mental health conditions came to a similar conclusion.

Citing such evidence, The New York Times editorial board recently recanted some of its earlier support for legalization. The newspaper wrote, “The unfortunate truth is that the loosening of marijuana policies … has led to worse outcomes than many Americans expected,” adding, “It is time to acknowledge reality and change course.”

The coalition of supporters frays

Still another issue is conflict within the legalization movement itself, particularly between the business and activist wings.

The tension between these groups is long-standing, with activists often accusing members of industry of being more focused on money than justice. And as the cannabis industry has grown, these tensions have become more acute.

In 2022, for example, the pro-cannabis organization True Social Equity in Cannabis sued three Illinois cannabis companies for engaging in coordinated anticompetitive practices and violating federal antitrust laws. In court documents, they called the three companies the “Chicago cartel,” before voluntarily dismissing the case.

Florida Gov. Ron DeSantis used a similar strategy in 2024 in his successful campaign against the legalization of marijuana for recreational use in the state. He consistently criticized “corporate cannabis,” a catchall phrase often used by critics to describe the large cannabis companies that increasingly dominate state markets. He warned voters that the law would create a “weed cartel.”

Prominent cannabis activists like former Massachusetts regulator Shaleen Title have also called out corporate cannabis in their accounts of what’s wrong with the legalization movement.

In many ways, these challenges are the result of the movement’s earlier success. Making marijuana legal has meant more people trying it, more people studying it and more people making money from it.

The insights from the past 12 years could help inform whatever comes next. The fact that public support for legalization remains high suggests that a return to the days of blanket prohibition is unlikely.

Still, as the history of cannabis law and policy has shown, there are no guarantees.

William Garriott’s work has been supported by the Wenner-Gren Foundation for Anthropological Research.

With the world struggling to get oil supplies moving from the Middle East, former House Speaker Newt Gingrich raised eyebrows with a social media post highlighting a radical idea: Use nuclear bombs to cut a new channel along a route that would avoid Iranian threats in the Strait of Hormuz.

Gingrich’s March 15, 2026, post linked to an article that labeled itself as satire. Gingrich has not clarified whether his endorsement was serious. But he is old enough to remember when ideas like this were not only taken seriously but actually pursued by the U.S. and Soviet governments.

As I discuss in my book, “Deep Cut: Science, Power, and the Unbuilt Interoceanic Canal,” the U.S. version of this project ended in 1977. At the time, Gingrich was launching his political career after working as a history and environmental studies professor.

Improving global trade and geopolitical influence

The idea for a new canal to move oil from the Middle East had emerged two decades earlier, in the context of another Middle East conflict, the Suez crisis. In 1956, Egypt seized the Suez Canal from British and French control. The canal’s prolonged closure caused the price of oil, tea and other commodities to spike for European consumers, who depended on the shipping shortcut for goods from Asia.

But what if nuclear energy could be harnessed to cut an alternative canal through “friendly territory”? That was the question asked by Edward Teller, the principal architect of the hydrogen bomb, and his fellow physicists at the Lawrence Radiation Laboratory in Livermore, California.

President Dwight D. Eisenhower’s administration had already begun promoting atomic energy to generate electricity and to power submarines. After the Suez crisis, the U.S. government expanded plans to harness “atoms for peace.”

Project Plowshare advocates, led by Teller, sought to use what they called “peaceful nuclear explosions” to reduce the costs of large-scale earthmoving projects and to promote national security. They envisioned a world in which nuclear explosives could help extract natural gas from underground reservoirs and build new canals, harbors and mountainside roads, with minimal radioactive effects.

To kick-start the program, Teller wanted to create an instant harbor by burying, and then detonating, five thermonuclear bombs in an Indigenous village in coastal northwestern Alaska. The plan, known as Project Chariot, generated intense debate, as well as a pioneering environmental study of Arctic food webs.

Teller and the Livermore physicists also worked with the Army Corps of Engineers to study the possibility of using nuclear explosions to build another waterway in Panama. Fearing that the aging Panama Canal and its narrow locks would soon be rendered obsolete, U.S. officials had called for building a wider, deeper channel that wouldn’t require any locks to raise and lower the ships along its route.

A sea-level canal would not only fit bigger vessels; it would also be simpler to operate than the lock-based system, which required thousands of employees. Since the early 1900s, U.S. canal workers and their families had lived in the Canal Zone, a large strip of land surrounding the waterway. Panamanians increasingly resented having their country split in two by the racially segregated, colony-like zone.

Crossing Central America

Nuclear explosions appeared to make a new sea-level canal financially feasible. The greatest impetus for the so-called Panatomic Canal occurred in January 1964, when violent anti-U.S. protests erupted in Panama. President Lyndon B. Johnson responded to the crisis by agreeing to negotiate new political agreements with Panama.

Johnson appointed the Atlantic-Pacific Interoceanic Canal Study Commission to determine the best site to use nuclear explosions to blast a seaway between the two oceans. Funded by a $17.5 million congressional appropriation – the equivalent of around $185 million today – the five civilian commissioners focused on two routes: one in eastern Panama and the other in western Colombia.

The Panamanian route spanned forested river valleys of the Darién isthmus and reached 1,100 feet above sea level. To excavate this landscape, engineers proposed setting off 294 nuclear explosives along the route, in 14 separate detonations, using the explosive equivalent of 166.4 million tons of TNT.

This was a mind-blowing amount of energy: The most powerful nuclear weapon ever tested, the Soviet “Tsar Bomba” blast in 1961, released the energy equivalent to 50 million tons of TNT.

To avoid the radioactivity and ground shocks, planners estimated that approximately 30,000 people, half of them Indigenous, would have to be evacuated and resettled. The canal commission considered this a formidable but not impossible obstacle, writing in its final report, “The problems of public acceptance of nuclear canal excavation probably could be solved through diplomacy, public education, and compensating payments.”

A not-so-hot idea, in retrospect

As explored in my book, marine and evolutionary biologists of the late 1960s sought to study the project’s less obvious environmental effects. Among other potential catastrophes, scientists warned that a sea-level canal could unleash “mutual invasions of Atlantic and Pacific organisms” by joining the oceans on either side of the isthmus for the first time in 3 million years.

Plans for the nuclear waterway ended by the early 1970s, not over concerns about marine invasive species but rather due to other complex issues. These included the difficulties of testing nuclear explosions for peaceful purposes without violating the Limited Nuclear Test Ban Treaty of 1963 and the huge budget deficits caused by the Vietnam War.

Despite the geopolitical and financial constraints, the sea-level canal studies employed hundreds of researchers who increased knowledge of the isthmus and its human and nonhuman inhabitants. Ironically, the studies revealed that wet clay shale rocks along the Darién route meant nuclear explosives might not work well there.

But for Project Plowshare’s biggest proponents, atomic excavation remained a worthwhile goal. In 1970, in their final report, the canal commissioners predicted that “someday nuclear explosions will be used in a wide variety of massive earth-moving projects.” Teller shared their commitment, as he explained near the end of his life in the 2000 documentary “Nuclear Dynamite.”

Today, given widespread awareness of the severe environmental and health effects of radioactive fallout, it is hard to envision a time when using nuclear bombs to build canals seemed reasonable. Even before Gingrich’s post sparked ridicule, press accounts described Project Plowshare using words like “wacky,” “insane” and “crazy.”

However, as societies struggle with disruptive new technologies such as generative AI and cryptocurrency, it is worth remembering that many ideas that ended up discredited once seemed not only sensible but inevitable.

As historians of science and technology point out, technological and scientific developments cannot be separated from their cultural contexts. Moreover, the technologies that become part of people’s daily lives often do so not because they are inherently superior, but because powerful interests champion them.

It makes me wonder: Which of the high-tech trends being promoted by influencers today will amuse, shock and horrify our descendants?

Christine Keiner received funding from the National Endowment for the Humanities, Lyndon Baines Johnson Foundation, and Eisenhower Foundation for the initial stages of this research.