Alcohol makes you drunk by crossing from your bloodstream into your brain and disrupting the way nerve cells talk to each other. Specifically, it amplifies inhibitory signals that quiet brain activity while dampening excitatory signals that keep you alert and coordinated. The result is the familiar cascade of loosened inhibitions, slurred speech, impaired balance, and foggy memory. But the full picture involves a chain of events that starts the moment a drink reaches your stomach and unfolds differently depending on your sex, your genetics, and even what you had for dinner.
From Glass to Bloodstream
Ethanol, the type of alcohol in drinks, is a small, water-soluble molecule that your body absorbs remarkably fast. Some absorption happens in the stomach, but the majority takes place in the upper small intestine, where the surface area is enormous. A study using radioactive tracers to measure absorption rates found that the stomach accounted for roughly 30 percent of absorption when alcohol was taken with food and only about 10 percent when taken with water alone.1PubMed. Use of measurements of ethanol absorption from stomach and intestine to assess human ethanol metabolism That difference matters because food slows gastric emptying, the rate at which your stomach pushes its contents into the intestine. Drink on an empty stomach and ethanol floods into the small intestine quickly, causing a sharper spike in blood alcohol. Eat a full meal first and the spike is blunted and delayed.
Once absorbed, ethanol dissolves into total body water and is carried by the blood to virtually every organ. The brain is especially vulnerable because it receives a large share of blood flow and because ethanol crosses the blood-brain barrier with ease. Within minutes of your first sip, concentrations in the brain begin to rise, and the effects you recognize as “being drunk” start to take hold.
How Alcohol Rewires Brain Signals
Your brain runs on a balance between excitatory signals (which make neurons fire) and inhibitory signals (which tell neurons to stay quiet). Alcohol tips that balance hard in the direction of inhibition, and it does so through at least two major pathways at the same time.
The first involves GABA, the brain’s main inhibitory chemical messenger. When GABA binds to its receptor, it calms neuronal activity. Alcohol enhances this effect by changing the shape of the GABA-A receptor so that GABA works more powerfully than usual.2PubMed Central. The role of GABAA receptors in mediating the effects of alcohol in the central nervous system This is the same receptor targeted by benzodiazepines and general anesthetics, which is why heavy intoxication can feel like sedation and why mixing alcohol with those drugs is so dangerous.3PubMed Central. The role of GABA(A) receptors in the development of alcoholism
The second pathway involves glutamate, the brain’s main excitatory messenger. Glutamate normally activates a receptor called the NMDA receptor, which is critical for alertness, learning, and memory formation. Even low concentrations of alcohol can block the NMDA receptor’s excitatory activity.4PubMed Central. Alcohol and glutamate. Experiments on brain slices have confirmed that ethanol directly reduces NMDA-mediated signaling between neurons.5PubMed. Reduction of voltage-dependent currents by ethanol contributes to inhibition of NMDA receptor-mediated excitatory synaptic transmission So alcohol is simultaneously turning up the brain’s “quiet down” signal and turning down its “wake up” signal. The combined effect is a widespread slowing of neural processing.
Alcohol also triggers the release of endorphins and dopamine in reward-related brain areas, which is why the early stages of a drink often feel pleasurable and relaxing. In animal studies, a dose of ethanol induced a burst of beta-endorphin release in the nucleus accumbens, a key reward center.6PubMed. Effects of acute ethanol on beta-endorphin release in the nucleus accumbens of selectively bred lines of alcohol-preferring AA and alcohol-avoiding ANA rats That rush of feel-good chemistry is part of what makes alcohol reinforcing and why that first drink can feel so different from the fifth.
Losing Judgment, Coordination, and Memory
The brain does not shut down uniformly when you drink. Different regions are affected at different doses, and that staggered vulnerability explains why intoxication unfolds in stages.
The prefrontal cortex, the region behind your forehead that handles planning, impulse control, and social judgment, is especially sensitive to alcohol. Research has shown that alcohol reduces the activity of certain interneurons in the prefrontal cortex, weakening the “top-down” control that normally keeps impulsive behavior in check.7PubMed Central. Alcohol reduces the activity of somatostatin interneurons in the mouse prefrontal cortex: A neural basis for its disinhibitory effect? This is why a couple of drinks can make you louder, more talkative, and more willing to do things you would normally think twice about, well before you have trouble walking.
As blood alcohol rises further, the cerebellum takes a hit. This structure at the back of the brain coordinates fine motor movements and balance. Alcohol-related damage to cerebellar function is what produces the classic stumbling gait and clumsy hand movements of someone who has had too much.8PubMed Central. Alcohol and the Cerebellum: Effects on Balance, Motor Coordination, and Cognition.
At higher doses still, alcohol disrupts the hippocampus, a deep brain structure essential for forming new memories. This is the mechanism behind alcohol-related blackouts, those gaps in memory where you were awake and functioning but your brain simply stopped recording. The hippocampus relies heavily on the NMDA receptors that alcohol suppresses, and when signaling there falls below a critical threshold, new memories cannot be laid down.9PubMed Central. What happened? Alcohol, memory blackouts, and the brain You do not lose consciousness during a blackout; you lose the ability to remember what happened.
How Your Body Clears Alcohol
Your liver does the heavy lifting when it comes to breaking alcohol down. The process involves a chain of enzymes. Alcohol dehydrogenase converts ethanol into acetaldehyde, a toxic compound that is largely responsible for the unpleasant effects of heavy drinking. A second enzyme, aldehyde dehydrogenase, then converts acetaldehyde into acetate, which is relatively harmless and eventually broken down into carbon dioxide and water. Additional enzyme systems, including one called CYP2E1, pitch in when alcohol intake is high or chronic.10PubMed Central. Overview: how is alcohol metabolized by the body?
The rate of this process is fairly fixed. For most people, the liver clears roughly one standard drink per hour. You cannot speed it up with coffee, food, or a cold shower. When you drink faster than your liver can metabolize, blood alcohol concentration climbs and you get progressively more intoxicated. The toxic intermediate acetaldehyde also builds up, contributing to nausea, flushing, and headache even while you are still drinking.
Why the Same Drink Hits People Differently
Two people can share a bottle of wine and end up in very different states. Several factors explain the variation.
Sex is one of the biggest. Women tend to reach higher blood alcohol concentrations than men after drinking the same amount, even after adjusting for body weight. One reason is that women have a higher proportion of body fat and a lower proportion of body water, so the same dose of alcohol is distributed into less fluid, producing a more concentrated solution.11PubMed Central. Gender differences in moderate drinking effects Another reason is that women have significantly lower gastric alcohol dehydrogenase activity, meaning less alcohol is broken down in the stomach before it reaches the bloodstream. One landmark study found that women’s first-pass metabolism in the stomach was only about 23 percent of men’s.12PubMed. High blood alcohol levels in women. The role of decreased gastric alcohol dehydrogenase activity and first-pass metabolism
Genetics also plays a major role, most visibly in the alcohol flush response. Roughly a third of people of East Asian descent carry a variant of the aldehyde dehydrogenase gene called ALDH2*2, which dramatically slows the conversion of toxic acetaldehyde into harmless acetate.13PubMed Central. The Alcohol Flush Response The result is a rapid buildup of acetaldehyde after even a small amount of alcohol, causing facial flushing, a racing heart, and nausea. People who carry one copy of this variant show significantly higher peak blood acetaldehyde levels and greater increases in pulse rate compared to those with the normal version of the gene.14PubMed. ALDH2*2 but not ADH1B*2 is a causative variant gene allele for Asian alcohol flushing after a low-dose challenge: correlation of the pharmacokinetic and pharmacodynamic findings This is not just an inconvenience; the acetaldehyde accumulation is a known carcinogen, and people with ALDH2*2 who drink heavily face sharply elevated risks of esophageal cancer.
Beyond sex and genetics, body size, hydration status, recent meals, medications, and how quickly you drink all shift the equation. A large person with a full stomach who sips slowly may barely feel a drink that would leave a smaller, fasting person noticeably impaired.
The Diuretic Effect and Dehydration
Anyone who has had a few beers knows the frequent trips to the bathroom. Alcohol suppresses the release of vasopressin, the hormone that tells your kidneys to hold onto water. With vasopressin dialed down, your kidneys let more water pass through into urine. A controlled crossover trial in older men found that alcoholic wine and spirits both produced significantly higher cumulative urine output over four hours compared to their non-alcoholic equivalents.15PubMed Central. The Diuretic Action of Weak and Strong Alcoholic Beverages in Elderly Men: A Randomized Diet-Controlled Crossover Trial Interestingly, the difference evened out over 24 hours, suggesting the body compensates by retaining more water later. But in the short term, you are losing fluid faster than you are replacing it, which contributes to the thirst, dry mouth, and headache that accompany or follow intoxication.
What Alcohol Does to Sleep
People sometimes use a nightcap to fall asleep faster, and the science confirms that alcohol can shorten the time it takes to drift off. But the tradeoff is not worth it. A systematic review and meta-analysis found that even a low dose of alcohol, roughly two standard drinks, delays the onset of REM sleep and reduces how much REM sleep you get over the night.16PubMed. The effect of alcohol on subsequent sleep in healthy adults: A systematic review and meta-analysis This disruption gets worse as the dose goes up.
The pattern is characteristic: the first half of the night tends to feature deeper-than-usual slow-wave sleep, but the second half becomes fragmented and restless as alcohol is metabolized and its sedative effect wears off. REM sleep, which is important for memory consolidation and emotional regulation, rebounds in the second half of the night in choppy, disrupted bursts.17PubMed Central. The Effects of Alcohol on Quality of Sleep A narrative review encompassing multiple study designs confirmed this overall picture: alcohol consistently suppresses REM sleep, increases sleep fragmentation, and impairs breathing during sleep, especially in the second half of the night.18PubMed Central. Alcohol, Wine, and Sleep in Adults: Insights from a Narrative Review You might feel like you slept soundly, but the architecture of your sleep was compromised.
Tolerance and the Mellanby Effect
Regular drinkers often notice they need more alcohol to feel the same effects they used to get from less. This is tolerance, and it happens at multiple levels. Your liver upregulates the enzyme systems that metabolize alcohol, so it clears ethanol somewhat faster. Your brain also adapts by adjusting receptor sensitivity: GABA-A receptors become less responsive to alcohol’s enhancing effects, and NMDA receptors become more active to compensate for chronic suppression. The net result is that a habitual drinker may appear relatively sober at a blood alcohol level that would visibly impair a non-drinker.
There is also a lesser-known phenomenon called acute tolerance, sometimes called the Mellanby effect. You tend to feel more impaired at a given blood alcohol concentration when it is rising than at the same concentration when it is falling. In other words, the same number on a breathalyzer produces different levels of felt intoxication depending on whether you are on the way up or down. This has been studied by comparing subjective intoxication ratings at matching blood alcohol levels on the ascending and descending limbs of the curve.19PubMed Central. Association between Overall Rate of Change in Rising Breath Alcohol Concentration and the Magnitude of Acute Tolerance of Subjective Intoxication via the Mellanby Method The practical consequence is that people sometimes feel “sober enough” to drive when their blood alcohol is still dangerously high, because their brain has already begun to adjust within a single drinking session.
What Causes a Hangover
The morning after a heavy drinking session brings a familiar collection of misery: headache, nausea, fatigue, sensitivity to light, and a general feeling of being unwell. Despite how common hangovers are, the science behind them is surprisingly messy. Multiple overlapping mechanisms contribute, and no single one fully explains the experience.
Researchers have identified several direct ways alcohol promotes hangover symptoms: its effects on urine production (leading to dehydration), disruption of the gastrointestinal tract, drops in blood sugar, disturbed sleep patterns, and interference with biological rhythms. On top of those, withdrawal-like effects as alcohol leaves the system, the buildup of acetaldehyde during metabolism, and biologically active compounds in drinks other than ethanol itself (called congeners) all appear to play a role.20PubMed Central. Alcohol hangover: mechanisms and mediators. Congeners are more abundant in darker spirits like bourbon and red wine than in clear spirits like vodka, which is one reason darker drinks have a reputation for producing worse hangovers.
Personality traits and family history of alcoholism have also been flagged as factors that may influence hangover severity, suggesting that susceptibility is not purely about how much or what you drank.
An Evolutionary Relationship with Alcohol
Humans did not invent alcohol. Yeasts fermenting sugars in ripe and overripe fruit have been producing ethanol for millions of years, and our primate ancestors were exposed to it long before anyone figured out how to brew beer. The “drunken monkey” hypothesis proposes that our attraction to alcohol is rooted in an ancient evolutionary link between the sugars of ripe fruit, the alcoholic fermentation carried out by wild yeast, and the feeding habits of our ancestors.21PubMed Central. Human Evolution and Dietary Ethanol
This is not just speculation. A study measuring ethanol concentrations in the fruits eaten by wild chimpanzees found that ripe fruit pulp from 20 plant species contained an average of about 0.3 percent ethanol by weight. Given that chimpanzees eat roughly 4.5 kilograms of fruit per day, the researchers estimated a daily ethanol intake of about 14 grams, equivalent to roughly 1.4 standard drinks.22PubMed Central. Ethanol ingestion via frugivory in wild chimpanzees Chimpanzees are not getting drunk from this, but they are consistently ingesting ethanol as a normal part of their diet. Genomic evidence suggests that a key mutation allowing more efficient ethanol metabolism appeared in our lineage around 10 million years ago, coinciding with a shift to more ground-based, fruit-rich foraging. Our livers, in other words, have been preparing for happy hour for a very long time.
Getting Drunk Without Drinking
In rare cases, people become intoxicated without consuming a drop of alcohol. Auto-brewery syndrome is a condition in which an overgrowth of fermenting microorganisms, typically yeast, in the gastrointestinal tract converts dietary carbohydrates into ethanol inside the body.23Livers. Endogenous Alcohol and Auto-Brewery Syndrome Complicating Liver Transplantation: A Case Report and Literature Review People with this condition can register measurably elevated blood alcohol levels after eating bread, pasta, or other starchy foods, and they experience genuine symptoms of intoxication including slurred speech, impaired coordination, and brain fog.
The condition is most often documented in people with gut dysbiosis, frequently following antibiotic use or in patients with underlying gastrointestinal disorders. It has also been reported as a complication after liver transplantation, when immune-suppressing medications create conditions that allow yeast to proliferate. Diagnosis is tricky because the symptoms mimic alcohol misuse, and patients are sometimes disbelieved by clinicians and family members. Treatment usually involves antifungal medications and a strict low-carbohydrate diet to starve the fermenting organisms. Auto-brewery syndrome is vanishingly rare, but it illustrates an interesting point: the biochemical machinery for producing and responding to ethanol is deeply embedded in biology, not something humans invented along with distillation.