Let's take a typical Friday evening. A person has a glass of wine and, within twenty minutes, notices their muscles feel a bit more relaxed, anxiety has subsided, and conversation flows more easily. An hour later, coordination is worse, speech is slower, and judgment is less sound. Two hours later, the picture can be completely different. What happens inside the brain during this time? Not in the abstract sense that “alcohol is bad,” but specifically: what molecules, what structures, what processes?
That's exactly what we're going to break down now.
Step One: Ethanol Enters the Bloodstream and Reaches the Brain
Ethanol is the “alcohol” in alcoholic beverages. It's a chemically simple molecule: two carbon atoms, one oxygen, and a few hydrogens. But its very simplicity is what makes it dangerous – it dissolves easily in both water and fats, meaning it effortlessly penetrates cell membranes.
After a person takes their first sip, ethanol is primarily absorbed in the small intestine – about 80% of it, with the rest absorbed directly through the stomach walls. The absorption rate depends on whether there's food in the stomach: fatty food slows the process, while carbonated drinks speed it up (carbon dioxide irritates the lining and increases blood flow). This is why champagne “hits” you faster than a table wine of the same strength.
From the intestine, ethanol enters the bloodstream and reaches the brain in about five minutes. This isn't a metaphor – literally five minutes, and the molecule is already there. The brain is an organ with a massive blood supply (about 15% of the body's total blood volume passes through it at rest), and ethanol gains almost direct access to it through the blood-brain barrier. This barrier is a system of cells that filters out most foreign substances. But it lets ethanol pass through with almost no resistance.
Step Two: What Ethanol Does to Neurons
This is where it gets interesting. The brain isn't a uniform mass but a complex system with different structures, each responsible for something specific. And ethanol doesn't act as a “general depressant” – it interferes with the operation of specific chemical channels.
GABA and “Braking the Brakes”
The first target is GABA (gamma-aminobutyric acid) receptors. GABA is the brain's main inhibitory neurotransmitter. When it binds to its receptor, a neuron gets the signal: “don't fire, slow down.” Ethanol enhances the effect of GABA – it's as if it's pressing the brake pedal harder than usual.
The result: reduced anxiety, muscle relaxation, and mild euphoria in the initial stage. This is why alcohol is subjectively perceived as “relaxing” – it literally suppresses the nervous system's activity. The first areas to be “braked” are those responsible for restraint and self-control – the prefrontal cortex. This leads to the well-known effect: a person becomes more talkative, bolder, and less inclined to think about consequences. This isn't liberation in a psychological sense – it's the filter being switched off.
NMDA Receptors and Inhibiting Excitation
The second target is NMDA (N-methyl-D-aspartate) receptors. These are receptors for glutamate, the main excitatory neurotransmitter. If GABA is the brake, glutamate is the gas. Ethanol blocks NMDA receptors, thereby weakening nervous system excitation.
It is through this mechanism that alcohol affects memory. NMDA receptors are critical for forming new memories – a process neurobiologists call long-term potentiation. When these receptors are blocked, the brain continues to perceive information but doesn't record it properly. This is why a person can't remember what they did the night before after heavy drinking – not because they “forgot,” but because the recording was never properly made.
At very high concentrations of ethanol, the blockade of NMDA receptors becomes so pronounced that a person enters a state close to anesthesia. It's no coincidence that early surgeons sometimes used alcohol as an anesthetic – the mechanism is the same, but the dose was incompatible with normal life.
Dopamine: Why It Feels Good
The third target is the dopamine system. Ethanol stimulates the release of dopamine in the nucleus accumbens, a structure central to the brain's reward system. Dopamine isn't the “happiness hormone” in the everyday sense; it's a signal that says, “this is important, repeat this.”
This is where the potential for addiction begins. The brain receives a strong dopamine signal, associates it with a specific behavior – drinking alcohol – and begins to form stable neural patterns that will later be triggered by familiar cues: the smell of wine, a certain place, company, or time of day. This isn't a weakness of will – it's chemistry, literally rewiring the neural circuits.
Step Three: What Happens in Different Brain Structures
The brain is not a monolith. Alcohol affects its various parts differently, and the effects layer on top of each other depending on the concentration of ethanol in the blood.
Prefrontal Cortex
Responsible for planning, decision-making, impulse control, and social behavior. It's one of the first structures to be “hit” by alcohol. This is why even with a low level of intoxication, a person is worse at assessing risks, says things they later regret, and makes decisions they wouldn't make when sober.
Cerebellum
Coordinates movement, balance, and motor precision. The cerebellum is sensitive to ethanol, and coordination impairments – staggering, unsteadiness, slurred speech – are its response. The neurons in the cerebellum are particularly densely packed and highly active, making them vulnerable to chemical interference.
Hippocampus
A key structure for memory and spatial orientation. This is where short-term memory is converted into long-term memory. Alcohol disrupts the hippocampus's function through the aforementioned blockade of NMDA receptors. With systematic use, the volume of the hippocampus can decrease – a finding documented in numerous neuroimaging studies.
Hypothalamus and Brainstem
The hypothalamus regulates basic bodily functions: temperature, appetite, and hormonal balance. Alcohol suppresses the production of antidiuretic hormone (vasopressin). The result: frequent urination and dehydration, which turn into a headache and dry mouth the next day.
The brainstem controls vital functions: breathing, heart rate, and the vomit reflex. At very high concentrations of alcohol, suppression of the brainstem can lead to respiratory arrest – the mechanism of death in alcohol poisoning.
Step Four: Concentration Matters
The level of intoxication is measured by blood alcohol concentration (BAC) – in promille (‰) or, as is common in some countries, in mg/100 ml. Here is what the picture looks like by level:
- 0.2–0.5‰ – Mild relaxation, reduced anxiety, slight impairment of fine motor skills. Many people at this level subjectively feel “better” without noticing the impairments.
- 0.5–1.0‰ – Noticeable impairment of coordination and reaction time, reduced critical thinking, emotional instability. In Germany, for example, the legal limit for drivers is 0.5‰ precisely because above this threshold, the ability to operate a vehicle is objectively diminished.
- 1.0–2.0‰ – Impaired coordination, slurred speech, significant memory blackouts, blurred vision. CNS depression increases.
- 2.0–3.0‰ – Severe intoxication, possible loss of consciousness. The gag reflex, as a protective mechanism, may either be triggered or suppressed.
- Above 3.0–4.0‰ – Life-threatening. Suppression of the respiratory center, possible coma, respiratory arrest.
It's important to understand that these figures are averages. Individual sensitivity depends on body weight, sex, metabolic rate, alcohol tolerance, and genetic factors – we'll return to this.
Step Five: Metabolism – How the Brain 'Recovers' from Alcohol
Ethanol is processed mainly in the liver. The key enzyme is alcohol dehydrogenase (ADH). It oxidizes ethanol to acetaldehyde – a substance that is toxic in its own right and causes many unpleasant symptoms: facial flushing, rapid heartbeat, nausea. Next, acetaldehyde is broken down by another enzyme – aldehyde dehydrogenase (ALDH) – into acetic acid, which is then “burned” like regular fuel.
The liver processes about one standard unit of alcohol per hour. One standard unit is, for example, 330 ml of 5% beer or 100 ml of 12% wine. It can't go any faster: neither coffee, nor a cold shower, nor physical activity can speed up this process. This is one of the most persistent myths about alcohol. The only thing that works is time.
While the liver is working, the alcohol concentration in the blood gradually drops, and the brain returns to its normal state. But not immediately and not completely – especially in cases of systematic use.
Step Six: Why Some People Get Drunk Faster Than Others
This isn't a matter of “willpower” or “habit” in the everyday sense. Several specific factors are at play.
Enzyme Genetics
The activity of alcohol dehydrogenase and aldehyde dehydrogenase varies greatly among individuals and is genetically determined. Carriers of certain variants of the ALDH2 gene – common among people of East Asian descent – accumulate acetaldehyde faster, causing pronounced facial flushing, tachycardia, and nausea even from small doses. This is a biological “defense” – and at the same time, a marker of increased risk for certain diseases with regular consumption.
Sex and Body Composition
Women generally have less total body water relative to body mass, and lower ADH activity in the stomach. This means that for the same amount of alcohol consumed, a woman's blood alcohol concentration will be higher. This isn't a stereotype – it's physiology.
Tolerance and Brain Adaptation
With regular use, the brain adapts: it reduces the sensitivity of GABA receptors and increases the number of NMDA receptors in an attempt to restore balance. This is the biochemical basis of tolerance – a person needs more alcohol to achieve the same effect. The downside: upon abrupt cessation of alcohol intake, the brain finds itself in a state of hyperexcitation – there are too many NMDA receptors and not enough inhibition. This is what causes withdrawal syndrome: anxiety, tremors, and in severe cases, seizures and delirium. This syndrome can be life-threatening and requires medical supervision.
Step Seven: Long-Term Consequences
Chronic alcohol use leaves structural marks on the brain. Neuroimaging – MRI and PET scans – allows us to see them quite clearly.
Reduction in Gray and White Matter Volume
Studies have documented a reduction in gray matter volume in the prefrontal cortex and hippocampus in people with chronic alcoholism. White matter – the bundles of axons through which neurons “talk” to each other – also suffers. This manifests as a decrease in information processing speed, impaired working memory, and executive function deficits.
Thiamine Deficiency and Wernicke-Korsakoff Syndrome
Alcohol impairs the absorption of thiamine (vitamin B₁) in the intestine. Thiamine is essential for normal metabolism in neurons. Its deficiency leads to severe brain damage – Wernicke-Korsakoff syndrome. This condition includes impaired eye movements, coordination problems, confusion, and, in its chronic phase, severe memory impairment where the person cannot form new memories. Some of these impairments are reversible with timely thiamine treatment, but not all.
Neuroinflammation
Another mechanism of chronic harm is the activation of microglia, the brain's immune cells. Chronic alcohol use triggers chronic inflammation in the nervous tissue. This neuroinflammation itself damages neurons and exacerbates degenerative processes.
Step Eight: Is the Damage Reversible?
Yes, partially – and this is an important part of the answer. The brain has significant plasticity, and upon quitting alcohol, some of the changes are reversed. Studies show that the volume of the hippocampus can partially recover within a few months of abstinence. Cognitive functions – attention, working memory, reaction speed – also show improvement.
But “partially” is the key word. The longer and more intense the use, the lower the chances of a full recovery. Some neural circuits lost due to atrophy or neuroinflammation do not recover. White matter recovers more slowly and not completely. Wernicke-Korsakoff syndrome in its chronic stage is virtually irreversible.
This doesn't mean that recovery is impossible – it is very real. It means that it is not limitless.
Conclusion: What We Have on the Table
Let's go through the chain one more time, but quickly.
- Ethanol enters the bloodstream, bypasses the blood-brain barrier, and reaches the brain in minutes.
- It enhances the inhibitory neurotransmitter GABA and blocks the excitatory receptor NMDA, resulting in relaxation, reduced anxiety, and memory impairment.
- The dopamine system registers a “reward” and forms neural patterns that can become the basis of addiction.
- Different brain structures react differently: the prefrontal cortex with reduced control, the cerebellum with impaired coordination, the hippocampus with memory disruption, and the brainstem – at high doses – with suppression of life-support functions.
- Metabolism occurs in the liver at a rate of about one unit per hour – and no other way.
- Individual differences in the speed of intoxication are explained by enzyme genetics, sex, body composition, and developed tolerance.
- Chronic use leaves structural marks: reduced gray and white matter, neuroinflammation, and thiamine deficiency.
- Recovery is possible, but limited – the brain's plasticity is real, but not infinite.
That's how it works. No moralizing, just maximum precision.
