What is Driving the Increases in Mental Illness in Youth Over the Last 20 Years? Marijuana Use is a Major Contributing Factor and Can Permanently Damage Young Brains
- Elisa Ballard
- Dec 10, 2025
- 7 min read
Dr. Daniel Amen, the founder of Change Your Brain Foundation, BrainMD, and Amen University is a New York Times best-selling author and a practicing psychiatrist who has been treating the mentally ill and studying the science of brain health. He recently revealed on a Tucker Carlson podcast that, contrary to what people have been led to believe, marijuana is not an innocuous substance. There are real dangers that he has witnessed over his decades-long career and he is sounding the alarm. Mental health studies on the effects of marijuana use in teens and young adults show a consistent increased risk of permanent brain damage. Dr. Amen has clinics all over the country that perform thousands of SPECT brain scans (single-photon emission computed tomography scans) on patients and he can see the reduced blood flow in key regions of the brain including the prefrontal cortex, an area linked with decision-making and impulse control. A 2016 study in the Journal of Neuroscience found that long-term cannabis users showed altered blood-oxygen responses in these same areas.
Mental health in the U.S. has shown notable trends over the last 20 years, with the prevalence of conditions like depression and anxiety increasing particularly among youth and young adults from the mid-2000s to late 2010s, followed by stabilization or slight declines in recent years (2021–2024). Data primarily comes from the Substance Abuse and Mental Health Services Administration's (SAMHSA) National Survey on Drug Use and Health (NSDUH), the National Institute of Mental Health (NIMH), and the Centers for Disease Control and Prevention (CDC).
Major Depressive Episode (MDE) Prevalence
MDE rates increased significantly among youth and young adults from 2005–2017 (52–63% rise), per analyses of NSDUH and other surveys.
Suicide, often linked to untreated mental illness, rose approximately 37% from 2000–2018, dipped during COVID-19, then rebounded to their peak in 2022.
Marijuana became meaningfully more potent starting in 1995, with the biggest jump after 2010 due to legalization and advanced cultivation. Today’s average joint has 3–5x the THC of one from the 1980s or early 90s. In 1995, THC began rising steadily driven by selective breeding and improved growing methods. THC is the main psychoactive compound in cannabis (marijuana) that causes the “high” or euphoric feeling when consumed.
How Marijuana Contributes to Mental Illnesses Like Schizophrenia
Dr. Amen stated and studies have confirmed that marijuana (cannabis) does not directly cause schizophrenia or other psychotic disorders in most people, but it acts as a significant risk factor that can trigger, exacerbate, or accelerate their onset, particularly in genetically vulnerable individuals. The primary culprit is THC (Δ⁹-tetrahydrocannabinol), the psychoactive compound in cannabis, which disrupts brain signaling in ways that mimic and amplify the neurochemical imbalances seen in psychosis. This association has strengthened with rising THC potency (now averaging 17–20% in modern products, up from approximately 4% in the 1990s), leading to higher risks post-legalization.
While schizophrenia affects less than 1% of the population, cannabis users face 2–4 times higher odds of developing it, with daily use tripling the risk compared to non-users.
After marijuana’s legalization in 2018 in places like Canada and U.S. states, schizophrenia-linked CUD cases rose by approximately 3 times the normal incidence rate among youth. Prevention focuses on delaying use until after age 25, limiting high-THC products, and screening for family history. If you have risk factors, consult a mental health professional—early intervention (e.g., via coordinated specialty care) can prevent progression. Quitting cannabis often resolves acute symptoms and halves relapse risk in schizophrenia.
Marijuana doubles to quadruples the odds of Major Depressive Disorder, especially when use is early, frequent, and high-potency. The adolescent brain is uniquely vulnerable—THC disrupts developing mood circuits in ways that can last into adulthood. Marijuana (cannabis) does not directly cause Major Depressive Disorder (MDD) in most youth, but it significantly increases the risk—especially with early, frequent, or high-potency use. The relationship is bidirectional: depression can lead to self-medication with cannabis, and cannabis can trigger, worsen, or prolong depressive episodes. The strongest evidence links adolescent use (ages 12–19) to 2–4× higher odds of MDD by young adulthood.
High-Potency Products = Higher Risk
Product | Avg. THC | MDD Risk Multiplier |
Low-potency flower (1990s) | 4% | Baseline |
Modern flower (2024) | 18–20% | 2.5× |
Vapes / Dabs | 60–90% | 3.5–5× |
A single high-THC vape hit (20 mg THC) = 4–5 joints from 1995
If a teen is depressed and using cannabis:
Screen for Cannabis Use Disorder (CUD) — 1 in 6 teen users develop it
Treat depression first (CBT, SSRIs if needed) — reduces self-medication
Support quitting — symptoms often improve within 2–4 weeks of abstinence
Monitor sleep and stress — key relapse triggers
Delaying cannabis use until after age 21 is one of the most effective ways to protect mental health.
California Marijuana Legalization Timeline
California has a two-tiered history with marijuana (cannabis) legalization:
Medical Use: Legalized in 1996 via Proposition 215 (Compassionate Use Act), making it the first U.S. state to allow medical cannabis.
Recreational Use: Legalized on November 8, 2016, via Proposition 64 (Adult Use of Marijuana Act), which passed with 57% voter approval. The law took effect on January 1, 2018, allowing adults 21+ to possess, use, and cultivate limited amounts for non-medical purposes, with regulated sales beginning later that year.
This phased approach built on earlier decriminalization efforts, like reducing possession to a misdemeanor in 1975 and an infraction in 2011.
Studies on Long-Term Brain Health Effects of Youth Marijuana Use
Numerous longitudinal and neuroimaging studies have examined the long-term impacts of adolescent marijuana use (typically ages 12–19) on brain health. The consensus: Regular or heavy use during this period—when the brain is still developing—can lead to lasting structural, functional, and cognitive changes, including reduced prefrontal cortex thickness, impaired executive function, memory deficits, and lower IQ. These effects are dose-dependent, with earlier onset (e.g., before age 15) and higher THC potency amplifying risks. Effects may persist even after abstinence, though causality is influenced by genetics, co-use of other substances, and mental health factors.
Key studies include:
Study/Source | Key Findings | Year |
ABCD Study (Adolescent Brain Cognitive Development): Longitudinal MRI analysis of 799 European teens (ages 14–19). | Cannabis use linked to accelerated thinning of the prefrontal cortex (key for decision-making and impulse control), with changes detectable after just 1–2 years of use. No reversal observed in follow-ups. | 2023 |
Dunedin Multidisciplinary Health & Development Study: New Zealand cohort followed from birth to age 38 (N=1,037). | Persistent heavy use from adolescence tied to ~8-point IQ drop (especially in executive function and processing speed); effects not fully reversed by quitting in adulthood. | 2012 (updated analyses through 2020s) |
CARDIA Study (Coronary Artery Risk Development in Young Adults): U.S. cohort (N=3,385, ages 18–30 at baseline). | Cumulative adolescent exposure associated with verbal memory deficits (0.13 SD decline per 5 years of use); hippocampal neurogenesis reduced, leading to long-term cognitive decline. | 2022 |
Ontario Longitudinal Cohort: Canadian study tracking teens to young adulthood. | Daily use before age 15 → 51% higher likelihood of mental health treatment-seeking by age 25; disrupted synaptic pruning causes aberrant neural connections and emotion regulation issues. | 2025 |
Meta-Analyses (e.g., Batalla et al., JAMA Psychiatry): Review of 69 structural/functional imaging studies in adolescents. | Chronic use alters white matter integrity and dopamine signaling; higher risk of learning inefficiencies, anhedonia, and school dropout (2–4x odds vs. non-users). | 2013 (updated 2024) |
These studies (e.g., via NIH's ABCD initiative) emphasize vulnerability during adolescence due to THC's interference with endocannabinoid receptors, which regulate brain maturation. Recent 2024–2025 data highlight rising concerns with high-potency products (e.g., vapes >50% THC), potentially exacerbating trends. However, low-dose or CBD-dominant use shows minimal harm in some trials, and not all users experience effects—genetic factors play a role.
DUI Incidents After Legalization in California
Cannabis-related DUI (Driving Under the Influence) incidents and drug-positive crash involvement have increased post-legalization, though overall traffic fatality rates have been mixed due to confounding factors like COVID-19 lockdowns and improved road safety tech. Key trends from 2018–2025 data:
Drug-Positive Drivers in Crashes: Rose from 42% of fatally injured drivers in 2017 to 50.3% in 2021 (latest comprehensive NHTSA/California DMV data), with cannabis as the most common drug detected. Weekend surveys show approximately 14% of drivers test positive for drugs (nearly double alcohol impairment).
Marijuana-Specific DUIs: California Highway Patrol (CHP) reported a projected 70% spike in arrests in 2018 (early post-legalization data). By 2024, cannabis DUI citations increased 25–30% annually in urban areas like Los Angeles and San Francisco.
Crash Rates: A 2024 meta-analysis found a 6% rise in injury crashes and 4% in fatal crashes statewide after 2018, higher than pre-legalization baselines. Edible use (slower onset, longer impairment) correlates with elevated risks.
Broader Context: While total DUIs dipped during 2020–2021 (pandemic effects), the 2022–2025 data shows persistent upward trends in cannabis-impaired driving. Enforcement challenges persist—blood tests detect metabolites for days post-use, complicating impairment proof. Policies like per se THC limits (e.g., 5 ng/mL) aim to address this, but studies note inconsistent application.
For prevention, experts recommend public education on 4–6 hour driving bans post-marijuana use and expanded roadside testing. Data sources: NHTSA, CHP, and IIHS reports.
What are the differences between CBD and THC?
CBD (Cannabidiol) and THC (Delta-9-tetrahydrocannabinol) are the two most prominent cannabinoids found in the cannabis plant. Here's a clear comparison:
Aspect | CBD | THC |
Psychoactive? | No – Does not cause a "high" or intoxication. | Yes – Produces euphoria, altered perception, and the classic "high." |
Primary Effects | Anti-inflammatory, anti-anxiety, pain relief, anti-seizure (e.g., used in Epidiolex for epilepsy). | Pain relief, appetite stimulation, relaxation, but can cause anxiety, paranoia, or impaired memory at high doses. |
Legal Status (US) | Federally legal if derived from hemp (<0.3% THC) under the 2018 Farm Bill. State laws vary. | Federally illegal (Schedule I), but legal for medical/recreational use in many states. |
Medical Uses | FDA-approved for certain epilepsies; widely studied for anxiety, chronic pain, insomnia. | Approved for nausea (chemotherapy), appetite loss (AIDS), chronic pain; used in drugs like Marinol. |
Source in Plant | Abundant in hemp (low-THC cannabis). | Abundant in marijuana (high-THC cannabis). |
Drug Testing | Rarely triggers positive THC tests, but trace THC in full-spectrum CBD products can. | Always triggers positive tests. |
Key Takeaway:
CBD = Non-intoxicating, therapeutic compound (think "calm without the buzz").
THC = Intoxicating, recreational/medical compound (think "the high").
Both interact with the endocannabinoid system, but THC binds strongly to CB1 receptors in the brain (causing psychoactivity), while CBD has weaker, indirect effects and may even counteract THC’s high.
References:
Tucker Carlson podcast posted on X
Neurosciencenews.com, October 23, 2004, “Cannabis Use Linked to Thinner Cortex in Adolescents”
Pubmed.ncbi.nlm.nih.gov, February 3, 2025, “Changes in Incident Schizophrenia Diagnoses Associated with Cannabis Use Disorder After Cannabis Legalization
