Category: Substance Use

What is alcohol? Alcohol is a small, simple molecule that moves quickly through almost every part of the body once it’s consumed. Shortly after consumption, alcohol levels increase in the brain, and signs of intoxication follow.

Your Brain on Alcohol

Alcohol slows brain activity by increasing the effects of GABA, the brain’s primary inhibitory neurotransmitter, while reducing glutamate, its main excitatory signal.

Glutamate helps activate the brain, while GABA helps calm it down.The two work together to keep brain activity balanced. When excitatory signals become too strong, inhibitory signals help bring things back under control.

Alcohol disrupts this balance by enhancing the calming effects and dampening the activating ones, which is why thinking, coordination, and reaction time are affected.

This combination can impair prefrontal cortex function within minutes of drinking. The prefrontal cortex—the part of the brain responsible for judgment, self-control, and decision-making—is especially affected by alcohol. As alcohol increases calming signals in this area, it essentially quiets the brain’s control center, which is why people often make choices while drinking that they wouldn’t make sober.

At the same time, alcohol overstimulates the brain’s reward system, flooding it with dopamine, reinforcing the urge to keep drinking. Over time, this makes everyday pleasures feel less satisfying by comparison.

The brain uses chemicals called neuromodulators. Unlike neurotransmitters, which send direct signals, neuromodulators adjust how those signals work—amplifying or dampening activity across larger brain systems.

Alcohol doesn’t just affect one pathway. It interacts with multiple systems at once, including those involved in mood, reward, stress, and relaxation. That’s part of what makes it so powerful—and so appealing.

Depending on the person and the moment, alcohol can feel like it’s doing different things: energizing you, calming you down, easing anxiety, or even changing how you perceive other people (including their attractiveness).

It’s also poison.

It can feel like more than one effect at once. But it’s still one substance acting on multiple systems in the brain.

Although alcohol is classified as a central nervous system depressant, it can feel stimulating at lower doses. Early effects may include increased heart rate and a sense of energy or alertness as inhibitory controls are reduced.

As it is absorbed and enters the brain, alcohol slows the central nervous system, which can feel like reduced anxiety, a lift in mood, and fewer inhibitions. At higher levels, alcohol leads to intoxication—slowed movement, poor coordination, slurred speech, and in extreme cases, stupor or coma. Alcohol disrupts how the brain communicates with itself.

Alcohol also affects the cerebellum, the part of the brain responsible for balance and coordination. Even small amounts can lead to slurred speech, blurred vision, slower reaction times, and impaired hand-eye coordination, increasing the risk of accidents and injury. It affects areas responsible for balance, memory, speech, and judgment, which is why injuries and poor decisions become more likely.

Over time, heavy drinking can actually change the brain, including shrinking the size of its cells. Chronic alcohol use is associated with reduced volume in the prefrontal cortex and hippocampus, compromising neural systems involved in executive functioning, impulse regulation, and memory consolidation.

Alcohol-related changes in the prefrontal cortex can worsen difficulties with stress regulation and emotional processing. Gradually, these changes may show up as patterns of behavior that are often mistaken for personality traits rather than effects on the brain.

Eventually, heavy drinking can physically change the brain, especially in areas responsible for judgment and memory. Brain scans show reduced gray matter in these regions, and the longer and heavier the drinking, the more pronounced the damage tends to be.

The hippocampus is affected in much the same way. This part of the brain helps turn experiences into long-term memories and supports navigation. With long-term alcohol use, it can shrink, making memory and learning more difficult.

Blackouts

Alcohol doesn’t just slow the brain down—it interferes with memory and learning. Glutamate helps the brain form new connections and store memories. When alcohol suppresses it, the brain has a harder time learning from what’s happening in the moment.

Alcohol-related memory gaps, or “blackouts” are not simply instances of forgetting, but periods in which memory encoding is disrupted. During these times, hippocampal functioning is impaired, preventing the formation of new memories. This is why individuals may recall part of an interaction but have no memory of how it concluded.

Alcohol in Your Body

Alcohol begins entering the bloodstream almost immediately. A small amount is absorbed through the mouth and esophagus, with additional absorption occurring in the stomach. From there, it moves into the bloodstream, where blood alcohol concentration (BAC) is measured.

BAC is expressed as a percentage. A BAC of 0.10% means there is one part alcohol for every 1,000 parts of blood.

Several factors influence how quickly BAC rises and how strongly alcohol affects you.

Body size plays a role. The more you weigh, the greater your blood volume, which can dilute alcohol and slow the rise in BAC. Body composition matters as well. Alcohol dissolves more easily in water than in fat, so individuals with a higher percentage of body fat tend to reach higher BAC levels more quickly.

Age also has an impact. As the body ages, alcohol is metabolized more slowly, which can lead to higher levels of alcohol remaining in the bloodstream. Physical health is another factor. Conditions that affect digestion can increase how quickly alcohol is absorbed.

What you eat—and what you mix alcohol with—matters. Food slows things down, keeping alcohol in the stomach longer so it enters the bloodstream more gradually. Carbonated drinks do the opposite. They speed up how quickly alcohol moves into the small intestine, where it’s absorbed faster, leading to a quicker and stronger effect.

Your emotional state even influences how alcohol affects you. When you’re acutely stressed, anxious, or fearful, the body shifts into a fight-or-flight response. Blood flow is redirected away from the digestive system, which can slow alcohol absorption. Chronic stress, however, has a different effect. Over time, it can disrupt digestive functioning, which may lead to faster absorption and stronger effects.

Gender differences also play a significant role in how alcohol is processed. Women don’t process alcohol as well, leading to increased health risks.

Alcohol is eliminated through the liver. A healthy liver can process approximately 0.5 ounces of pure alcohol per hour, which is about one drink (12 ounces of beer, 5 ounces of wine, or 1.5 ounces of liquor). The rest of the alcohol circulates through your bloodstream until the liver is ready to process it.

The liver breaks down alcohol through different pathways.

Most of the alcohol you drink is metabolized through the alcohol dehydrogenase (ADH) pathway, which occurs primarily in the liver and involves two main steps.

First, alcohol is converted into acetaldehyde by the enzyme alcohol dehydrogenase (ADH), with the help of NAD⁺ (nicotinamide adenine dinucleotide), a coenzyme derived from B vitamins. During this step, hydrogen atoms are removed from the alcohol molecule, forming acetaldehyde—a highly toxic compound that contributes to many of alcohol’s harmful effects.

Next, acetaldehyde is rapidly converted into acetate by another enzyme, aldehyde dehydrogenase (ALDH). This step also uses NAD⁺ and helps reduce the buildup of acetaldehyde in the body.

Acetate is then further broken down into carbon dioxide and water. Unlike the earlier steps, this final stage can occur in many tissues throughout the body, not just in the liver.

NAD⁺ plays a critical role in both steps of alcohol metabolism. As alcohol is processed, NAD⁺ is converted into NADH. When NADH accumulates, it disrupts the body’s normal metabolic balance. This shift can contribute to low blood sugar, increased fat production in the liver, and elevated uric acid levels, which may increase the risk of conditions like gout.

Although the MEOS (microsomal ethanol oxidizing system) is considered a secondary pathway for metabolizing alcohol, it becomes increasingly important with repeated or heavy drinking.

Unlike the primary pathway, the MEOS system becomes more active over time. As it ramps up, the body is able to break down alcohol more quickly. This contributes to tolerance, meaning a person can drink more and feel less of the effects.

This can be misleading. It may feel like the body is “handling alcohol better,” when in reality, it’s adapting in a way that often leads to increased consumption and greater long-term harm.

The MEOS pathway also produces toxic byproducts and places additional stress on the liver, which helps explain why heavy drinking can accelerate physical damage even as subjective effects seem to decrease.

Alcohol Poisoning

Alcohol poisoning or overdose happens when there’s so much alcohol in the bloodstream that the brain starts to shut down the systems that keep you alive—breathing, heart rate, and body temperature.

Signs include confusion, trouble staying awake, vomiting, seizures, slow or irregular breathing, a slow pulse, cold or clammy skin, and a loss of the gag reflex, which increases the risk of choking. Body temperature can also drop dangerously low.

This is a medical emergency. Without immediate help, alcohol overdose can cause permanent brain damage or death.

Higher blood alcohol concentration (BAC) levels are associated with more severe effects.

At 0.20–0.29%, people may experience confusion, vomiting, and blackouts.
At 0.30–0.39%, there is a high risk of unconsciousness and dangerously slowed or suppressed breathing.
At 0.40% and above, the risk of death becomes extremely high due to respiratory failure.

Conclusion

Alcohol is often marketed as a way to relax, celebrate, connect, or escape, but beneath those experiences is a drug that profoundly alters the brain and body. Its effects can feel pleasant in the moment, which is precisely why it can be so difficult to recognize the harm it causes.

Understanding how alcohol works is not about fear or judgment. It’s about clarity. The more we understand what alcohol actually does—from disrupting memory and decision-making to reshaping the brain over time—the better equipped we are to make informed choices about our relationship with it. Whether you drink occasionally, struggle with alcohol, or are simply curious, knowledge provides a foundation for understanding both the appeal of alcohol and the risks that often accompany it.

References

Ketcham, K., & Asbury, W. (2003). Beyond the influence: Understanding and defeating alcoholism. Bantam.

Nutt, D. (2020). Drink?: The new science of alcohol and your health. Yellow Kite.

Porter, W. (2015). Alcohol explained. Alcohol Explained Ltd.