As cannabis legalisation spreads across the United States, a growing body of research is challenging the widespread belief that marijuana is harmless. Long-term studies are now revealing significant health risks from long-term marijuana use that many users and employers need to understand.

The question came from a business owner in Oklahoma City, concerned about employees using medical marijuana: “What do we know about the short- and long-term effects of marijuana use?” Dr Hal Scofield’s answer might surprise those who consider cannabis a benign substance.

The Research Challenge

Understanding the consequences of chronic cannabis consumption has been notoriously difficult. Widespread legalisation has only occurred over the past decade, and federal restrictions in the United States continue to hamper research efforts. However, international studies and emerging evidence are beginning to paint a concerning picture.

Most people assume cannabis is safe. Yet as usage expands and researchers compile decades of data, that assumption is being seriously questioned by the scientific community.

Lung Cancer Risk Doubles for Heavy Users

One of the most comprehensive investigations comes from Sweden, where scientists tracked nearly 50,000 military draftees over 40 years, from 1969-70 until 2009. The men were between 18 and 20 years old when first assessed.

The findings were stark. Those classified as “heavy” marijuana smokers—defined as using cannabis more than 50 times over their lives—had more than twice the risk of developing lung cancer. This elevated risk persisted even after researchers adjusted for tobacco use and other factors that might contribute to disease.

This Swedish cohort has provided invaluable insights into health risks from long-term marijuana use that simply weren’t available before such extended research became possible.

Mental Health: A Troubling Connection

The same Swedish study revealed another alarming pattern. Participants who used marijuana showed a significantly elevated probability of developing schizophrenia and other psychoses. The increased risk remained even when scientists accounted for personality traits and the use of other psychoactive substances.

Subsequent research has reinforced these findings. A 2023 Danish analysis examined nearly 7 million people and determined that up to 30% of schizophrenia cases amongst young men were linked to cannabis use disorder. The sheer scale of this study—tracking millions of individuals—lends considerable weight to its conclusions.

Earlier in 2025, a study of more than 13 million Canadians found the same association between marijuana use and psychotic disorders. The consistency across different populations and research methodologies strengthens the evidence that these dangers of regular cannabis use are genuine and significant.

Cardiovascular Dangers Emerge

Research published in June 2025 in the journal Heart added another dimension to our understanding. The study found that cannabis use was associated with a twofold increase in the risk of death from cardiovascular disease.

This finding challenges the notion that marijuana affects only the lungs and brain. The cardiovascular system, it appears, is also vulnerable to the impact of chronic cannabis consumption.

Young Brains at Risk

Perhaps the most concerning findings involve adolescent brain development. A Johns Hopkins University study utilising functional magnetic resonance imaging (fMRI) found that long-term marijuana use during adolescence was linked to alterations in brain function during cognitive tasks.

The scientists noted that these changes appeared in portions of the brain related to executive functioning and emotional control—precisely the areas still developing during teenage years. The implications for decision-making, impulse control, and emotional regulation could be profound and long-lasting.

Another study determined that marijuana use is associated with higher levels of suicidal ideation amongst teenagers, adding urgency to concerns about youth cannabis consumption.

The Emergency Room Epidemic: CHS

Beyond long-term studies, physicians are witnessing a surge in acute problems. Dr Scofield notes that he and colleagues are seeing increasing numbers of patients experiencing significant health issues related to regular cannabis use, including psychosis, psychiatric disorders, and a condition virtually unknown until recently.

Cannabinoid hyperemesis syndrome (CHS) has emerged as a commonplace diagnosis in emergency departments. The condition causes chronic nausea and vomiting brought on by regular marijuana use. Until a few years ago, CHS was virtually unheard of; now it’s routine.

Interestingly, CHS patients find temporary relief from long, hot showers or baths. However, the only known cure is the one solution they least want to hear: stop using cannabis entirely. This illustrates one of the key dangers of regular cannabis use—symptoms that only resolve with complete cessation.

Implications for Employers and Society

The Oklahoma business owner’s concern about employee health reflects a broader societal question. As medical and recreational marijuana become increasingly accessible, understanding the health risks from long-term marijuana use becomes crucial for workplace safety, productivity, and employee wellbeing.

Employers in states with legal marijuana face complex challenges. They must balance employee rights with workplace safety concerns, particularly in industries where impaired judgement or slowed reaction times could prove dangerous.

The research suggests that regular cannabis use may affect cognitive function, mental health stability, and physical health in ways that could impact job performance and attendance. These aren’t abstract concerns—they’re measurable effects with real-world implications.

The Prevention Perspective

The emerging evidence on chronic cannabis consumption presents important considerations for public health policy and individual decision-making. Whilst some adults may choose to use marijuana despite these risks, ensuring they have access to accurate information about potential harms is essential.

Young people deserve particular attention. The brain development research suggests that adolescent cannabis use may have lasting effects on cognitive and emotional functioning. Education programmes that present scientific evidence about health risks from long-term marijuana use—rather than relying on scare tactics or outdated information—could help young people make informed choices.

For those already using cannabis regularly, awareness of warning signs becomes crucial. Symptoms such as persistent nausea, changes in mental state, respiratory problems, or cardiovascular issues warrant medical attention and honest discussion with healthcare providers about marijuana use.

The Knowledge Gap Persists

Despite these concerning findings, significant gaps remain in our understanding. Federal restrictions continue to limit research opportunities in the United States, where much of the legalisation is occurring. More studies examining diverse populations, different consumption methods (smoking versus edibles), and varying potency levels are needed.

The Swedish studies provide invaluable long-term data, but they examined cannabis use from decades ago when potency was typically lower than today’s products. Modern marijuana often contains significantly higher concentrations of THC (tetrahydrocannabinol), the primary psychoactive compound, raising questions about whether contemporary users face even greater risks.

A Changing Narrative

The widespread perception of marijuana as harmless is increasingly at odds with scientific evidence. Long-term studies are revealing effects that range from doubled cancer and cardiovascular disease risks to psychiatric disorders and a debilitating nausea syndrome that has physicians baffled.

As Dr Scofield’s response suggests, the “safe” label many apply to cannabis deserves serious reconsideration. The evidence doesn’t support a narrative of widespread harm affecting all users, but it does indicate that the dangers of regular cannabis use are real—risks that users, employers, policymakers, and healthcare providers need to understand.

For the Oklahoma employer concerned about workforce health, the research provides grounds for that concern. For individuals considering or currently using cannabis, the studies offer important information for weighing benefits against potential harms. And for society broadly, the emerging evidence suggests that legalisation policies should be accompanied by robust education, research funding, and support systems for those experiencing adverse effects.

The conversation about marijuana is evolving from ideology to evidence. That evolution, uncomfortable as it may be for cannabis advocates, ultimately serves everyone’s interests by ensuring decisions are made with eyes open to both benefits and risks.

Understanding health risks from long-term marijuana use isn’t about moral judgement—it’s about equipping people with the scientific knowledge they need to make informed decisions about their health and wellbeing.

Source: The Oklahoman