Alcohol and weed both reach the brain within minutes and change how its cells talk to each other, which is why each can blunt memory and judgment while you’re drunk or high. Their long-term risks point in different directions, though. Alcohol is mostly tied to physical loss of brain tissue, while weed is mostly tied to mental-health risks like psychosis. The brain is also more resilient than many people assume, and a good part of the harm from both starts to undo itself once use stops.
What Alcohol and Weed Do in the Brain While You’re Under the Influence
Alcohol turns down brain activity on two fronts. It strengthens GABA, the brain’s main “slow down” messenger, and blocks glutamate, its main “go” messenger.1PubMed. Glutamate and GABA receptor dysfunction in the fetal alcohol syndrome Signals between brain cells slow, which produces relaxation, slurred speech, poor coordination and dulled judgment. The front part of the brain, which handles planning and self-control, is especially sensitive to alcohol, which helps explain risky choices while drinking.
The effect shifts over a night. Early on, while blood alcohol is rising, alcohol is often more stimulating, so the first drink or two can feel lively and sociable. Later ones bring clumsiness, heavy drowsiness and slurred speech. Dulled judgment tends to arrive before you feel very drunk, so you can already be making poor decisions while feeling fine.
THC, the ingredient in weed that makes you high, latches onto CB1 receptors, docking points the brain normally uses for its own cannabinoid-like chemicals.2PubMed Central. The diverse CB1 and CB2 receptor pharmacology of three plant cannabinoids: delta9-tetrahydrocannabinol, cannabidiol and delta9-tetrahydrocannabivarin That reduces how much messenger chemical brain cells release, which ripples into the dopamine, serotonin, GABA and glutamate systems.3PubMed Central. The effect of cannabis use on memory function: an update It also interferes with the process brain cells use to store new memories, which is why short-term memory slips while you’re high.4PubMed Central. The effect of cannabis use on memory function: an update
THC only partly switches these receptors on, so its effect depends on how many receptors a person has and how active their own system already is.5Cambridge Core. How Cannabis Works in the Brain Tolerance matters too. Regular users feel less from the same amount, while a first-timer with an unprepared brain can feel much stronger effects.
Both substances feel rewarding for the same underlying reason. Like every drug that people come to crave, cannabis raises dopamine in the brain’s reward circuit.6PubMed Central. A Brain on Cannabinoids: The Role of Dopamine Release in Reward Seeking Over time, cues linked to the drug, like a lighter or a certain friend, can trigger that dopamine rise on their own.7PubMed Central. A Brain on Cannabinoids: The Role of Dopamine Release in Reward Seeking Both also make time and risk harder to judge, which matters a great deal for driving.
How Each One Affects Memory and Thinking
In teens who use heavily, each substance leaves a different fingerprint on thinking. Teens who binge drink score worse at switching between tasks, recalling words and reading.8Journal of the International Neuropsychological Society. Heavy Alcohol Use, Marijuana Use, and Concomitant Use by Adolescents Are Associated with Unique and Shared Cognitive Decrements Teens who use weed score worse at holding back impulses, remembering with hints and moving quickly.9Journal of the International Neuropsychological Society. Heavy Alcohol Use, Marijuana Use, and Concomitant Use by Adolescents Are Associated with Unique and Shared Cognitive Decrements Teens who do both show the drinkers’ problems and the weed users’ problems, plus weaker working memory, the ability to hold information in mind.10Journal of the International Neuropsychological Society. Heavy Alcohol Use, Marijuana Use, and Concomitant Use by Adolescents Are Associated with Unique and Shared Cognitive Decrements Weaker working memory makes mental math or following multi-step directions harder. In mice, cannabinoids impair working memory through receptors on the brain’s support cells, not on nerve cells themselves.11PubMed. Acute cannabinoids impair working memory through astroglial CB1 receptor modulation of hippocampal LTD
These findings describe groups. The teen studies compare heavy users with non-users, so they show average differences and can’t predict what will happen to any one person. The differences show up most clearly in heavy and frequent use, so an occasional user is in a different position from someone using daily or bingeing weekly. The studies also can’t always separate the effect of a substance from traits people already had, so they point to a pattern rather than proving that each drug caused each deficit.
Does Long-Term Use Shrink the Brain
With alcohol, more drinking means less brain tissue, even below addiction levels. Brains are made of gray and white matter: gray matter is the thinking tissue, and white matter is the wiring that carries signals between regions. In a large population of ordinary adults, people who drank more had less gray matter and white matter across the whole brain.12PubMed Central. Associations between alcohol consumption and gray and white matter volumes in the UK Biobank The link grew stronger with each extra daily unit of alcohol.13PubMed Central. Associations between alcohol consumption and gray and white matter volumes in the UK Biobank
Units add up faster than many people expect. One UK unit is 8 grams of alcohol, about half a pint of regular beer, and a large glass of wine can be about three units. Because the effect rises with units, cutting back helps even if you don’t quit.
In people with alcohol addiction, the losses show up in specific regions. People dependent on alcohol have less tissue in areas for movement, planning, memory and balance, including the cerebellum.14PubMed Central. A widespread distinct pattern of cerebral atrophy in patients with alcohol addiction revealed by voxel‐based morphometry Losing white matter disrupts communication between regions and over years can lead to alcohol-related dementia.15Dementia UK. What is alcohol related brain damage (ARBD)? That dementia is largely preventable by cutting down.16Dementia UK. What is alcohol related brain damage (ARBD)?
Warning signs include repeated memory gaps or blackouts, trouble planning, organizing or controlling impulses, and unsteadiness when walking. These are worth seeing a doctor about, since stopping early improves outcomes.
Heavy drinking activates the brain’s immune cells through a sensor on those cells.17PubMed Central. Pivotal role of TLR4 receptors in alcohol-induced neuroinflammation and brain damage This triggers inflammation and brain cell death; in mice lacking the sensor, alcohol caused far less damage.18PubMed Central. Pivotal role of TLR4 receptors in alcohol-induced neuroinflammation and brain damage
Weed’s structural picture is less clear. Long-term users often show changes in the hippocampus, the brain’s memory hub, but results vary between studies.19PubMed. Structural MRI findings in long-term cannabis users: what do we know? THC probably does change brain structure, especially in memory regions, though less dramatically than heavy drinking.
Weed, High Potency and the Risk of Psychosis
How strong the weed is and how often it’s used drive the risk. Among people with a first episode of psychosis who used cannabis, 78% used high-potency “skunk,” compared with 37% of healthy users.20PubMed Central. High-potency cannabis and the risk of psychosis Using skunk-type weed (about 14% THC) was linked to about three times the risk of psychotic disorders, rising to about five times with daily use.21PubMed Central. Adverse effects of heavy cannabis use: even plants can harm the brain Across Europe, daily use of weed above 10% THC was linked to about five times the risk.22PubMed Central. Adverse effects of heavy cannabis use: even plants can harm the brain In Amsterdam, where products can reach 22% THC or more, the risk was over nine times higher.23PubMed Central. Adverse effects of heavy cannabis use: even plants can harm the brain
“Five times the risk” compares users with non-users; it doesn’t mean five in ten users develop psychosis. Psychotic disorders are uncommon to begin with, so even a several-fold increase still leaves most users without one. The numbers come from studies comparing people with and without psychosis, which show a strong link but can’t by themselves prove weed caused each case.
Weed that balances THC with CBD looked safer. Hash with roughly equal THC and CBD carried no added psychosis risk in South London.24PubMed Central. Adverse effects of heavy cannabis use: even plants can harm the brain
On a dispensary label, keep in mind that many modern flower products exceed 15% THC and concentrates go far higher. Lower-THC products and those with more CBD appear to carry less psychosis risk, and some experts suggest choosing products clearly labeled under 15% THC. Unlabeled street weed gives no way to judge strength.
Some people should be especially cautious: those with a family history of schizophrenia or psychosis, and teens and young adults, whose brains are still developing. Anyone who has had paranoia or hallucinations while high should stop and talk to a doctor.
What Happens When You Mix Alcohol and Weed
Drinking first changes how much THC gets in. Drinking alcohol before smoking weed produces higher blood levels of THC than weed alone.25Addiction Center. Alcohol And Marijuana Alcohol is thought to make THC easier to absorb into the bloodstream, so someone who smokes their usual amount after a few drinks may get a stronger high than they’re used to, without changing anything about the weed. In a controlled trial, the combined effects on mood, thinking and coordination added together rather than multiplying.26PubMed. Separate and combined effects of alcohol and cannabis on mood, subjective experience, cognition and psychomotor performance: A randomized trial
Order matters: drinking then smoking boosts the high more than the reverse. Combined use worsens coordination, raising driving and fall risks. Nausea and “greening out” are common when the two stack. Waiting between them and using less of each lowers the risk.
Blood vessels may react more strongly to the pair. In lab tests on male animal brain arteries, alcohol plus THC narrowed them more than either alone.27PubMed Central. Cerebrovascular Effects of Alcohol Combined with Tetrahydrocannabinol Cannabis by itself, though, tends to widen brain blood vessels and increase blood flow.28PubMed Central. Alcohol & cannabinoid co-use: implications for impaired fetal brain development following gestational exposure
Why Teenage Brains Are Most at Risk
The parts of the brain still being built are the most exposed. The brain keeps maturing into the mid-20s, with the front region that controls planning finishing last. The cortex, emotion centers and cerebellum, which change most during the teen years, are especially vulnerable to alcohol.29PubMed Central. The effect of alcohol use on human adolescent brain structures and systems In animals, teen drinking led to thinner frontal regions, weaker wiring and more impulsive, risk-taking behavior in adulthood.30PubMed Central. Effect of alcohol use on the adolescent brain and behavior
Weed may disrupt key developmental steps. The brain’s own cannabinoid system helps guide development, so heavy teen weed use may disrupt the trimming of unused connections and the growth of white matter.31PubMed. Cannabis and adolescent brain development When talking with a teen, focus on concrete effects like memory, grades and impulse control, and explain that delaying first use matters because the brain is still building.
Can the Brain Recover After You Stop
Alcohol-related shrinkage partly reverses with sobriety. Brain volume regrows fastest in the first month without drinking.32PubMed. Temporal dynamics and determinants of whole brain tissue volume changes during recovery from alcohol dependence The people with the most shrinkage and heaviest drinking recovered the most volume, simply because they had the most to regain.33PubMed. Temporal dynamics and determinants of whole brain tissue volume changes during recovery from alcohol dependence People who stay sober regain tissue in the frontal lobes, cerebellum and white matter, while people who relapse do not.34PubMed Central. Deformation-based morphometry of brain changes in alcohol dependence and abstinence Some changes appear to be permanent.35PubMed Central. Magnetic resonance imaging of the living brain: evidence for brain degeneration among alcoholics and recovery with abstinence
Weed’s receptor changes bounce back within weeks. Daily users have fewer working cannabinoid receptors, which is part of how tolerance builds.36PubMed Central. Reversible and regionally selective downregulation of brain cannabinoid CB1 receptors in chronic daily cannabis smokers After about a month off, receptor levels returned to those of non-users.37PubMed Central. Cannabis Withdrawal: A Review of Neurobiological Mechanisms and Sex Differences In the hippocampus, the memory hub, recovery lagged.38PubMed Central. Reversible and regionally selective downregulation of brain cannabinoid CB1 receptors in chronic daily cannabis smokers A tolerance break of a few weeks can largely reset weed’s effects.
Sleep improves too. In the first days off, recent drinking affected dream sleep and recent weed use affected deep sleep in teens.39PubMed Central. Sleep Architecture in Adolescent Marijuana and Alcohol Users during Acute and Extended Abstinence After four weeks, neither drinking nor weed use made any difference to sleep.40PubMed Central. Sleep Architecture in Adolescent Marijuana and Alcohol Users during Acute and Extended Abstinence
The first month isn’t always comfortable. Weed withdrawal can bring irritability, poor sleep and vivid dreams for the first couple of weeks. Alcohol withdrawal in heavy drinkers can be dangerous and may need medical help.
Why Some People Are Hit Harder
Genes change how the brain reacts to weed. People with certain variants of cannabinoid-related genes show stronger reward-circuit activity when they see weed-related cues.41PubMed Central. Individual and additive effects of the CNR1 and FAAH genes on brain response to marijuana cues
Men and women differ as well. The brain areas most changed by substance problems, including reward and memory regions, differ between men and women.42PubMed Central. Sex Differences in Substance Use Disorders: A Neurobiological Perspective
CBD, a non-intoxicating part of cannabis, barely binds to the brain’s cannabinoid receptors on its own.43PubMed. The Impact of Cannabidiol on Human Brain Function: A Systematic Review When THC is present, though, it may dampen THC’s effect at those receptors.44PubMed. The Impact of Cannabidiol on Human Brain Function: A Systematic Review That may help explain why balanced THC-CBD hash carried less psychosis risk.