Alcohol, in the context most people mean, is ethanol: a naturally occurring compound produced when yeast ferments sugar. Its chemical formula is C2H6O, a simple molecule made of two carbon atoms, a few hydrogens, and a hydroxyl group (an oxygen-hydrogen pair) that defines it as an alcohol. This single compound is responsible for the intoxicating effects in every beer, glass of wine, and shot of liquor on the planet.
How Alcohol Is Made
Every alcoholic beverage starts with fermentation. Yeast, a single-celled organism, consumes simple sugars and produces ethanol and carbon dioxide as byproducts. Grapes provide the sugar for wine, barley for beer, agave for tequila, and so on, but the underlying process is identical. The yeast does the work; the raw ingredient determines the flavor.
Fermentation alone typically produces beverages with relatively low alcohol content, topping out around 12 to 15 percent before the rising ethanol concentration kills the yeast. Distilled spirits like vodka, whiskey, and rum go a step further: the fermented liquid is heated until the ethanol evaporates, then that vapor is collected and condensed. Because ethanol evaporates at a lower temperature than water, the resulting liquid is far more concentrated, commonly reaching 40 percent alcohol by volume or higher.
What Counts as One Drink
In the United States, a “standard drink” contains about 14 grams (roughly 0.6 fluid ounces) of pure ethanol. That amount is found in a 12-ounce can of regular beer at 5% alcohol, a 5-ounce glass of wine at 12%, or a 1.5-ounce shot of distilled spirits at 40%. These numbers matter because people often underestimate how much they’re actually consuming. A large pour of wine at a restaurant can easily be two standard drinks, and a strong craft beer at 9% alcohol is nearly double a standard serving.
Current CDC guidelines define moderate drinking as two drinks or fewer per day for men and one drink or fewer per day for women.
What Alcohol Does in Your Brain
Ethanol is classified as a central nervous system depressant, which means it slows down brain activity rather than speeding it up. The initial feelings of relaxation and lowered inhibition might seem stimulating, but that’s actually the result of your brain’s “braking system” being amplified.
The primary target is a receptor activated by GABA, the brain’s main calming neurotransmitter. Under normal conditions, GABA helps regulate how excitable your nerve cells are. Ethanol enhances this effect, making nerve cells even less likely to fire. At low doses, this produces a mild sense of ease. At higher doses, it progressively impairs coordination, judgment, memory, speech, and eventually consciousness. Ethanol also affects receptors for other chemical messengers in the brain, which contributes to its wide range of effects on mood, perception, and motor control.
Blood Alcohol Levels and Impairment
Blood alcohol concentration (BAC) is expressed as a percentage of ethanol in your bloodstream. Even small changes produce noticeable shifts in how you think and move:
- 0.01 to 0.05% BAC: Mild relaxation, slightly reduced alertness, and subtle changes in judgment. Many people feel “loosened up” but functional.
- 0.06 to 0.15% BAC: Slurred speech, reduced coordination, impaired judgment and memory. The legal driving limit in most U.S. states is 0.08%.
- 0.16 to 0.30% BAC: Difficulty walking and speaking, confusion, nausea, memory blackouts, vomiting, and possible loss of consciousness.
- Above 0.31% BAC: Life-threatening territory. Breathing can slow dangerously, and coma or death becomes a real risk.
These ranges are averages. Body weight, sex, food intake, hydration, tolerance, and the speed of drinking all influence where you fall on this spectrum for a given number of drinks.
How Your Body Breaks Down Alcohol
Your liver handles the vast majority of the work. The process happens in two main stages. First, an enzyme called alcohol dehydrogenase converts ethanol into acetaldehyde, a toxic compound that is a known carcinogen. Second, another enzyme converts acetaldehyde into acetate, a much less harmful substance your body can break down into water and carbon dioxide and eliminate easily.
That intermediate step is why drinking feels unpleasant for some people more than others. If your body is slow to clear acetaldehyde, it builds up, causing flushing, nausea, and a rapid heartbeat. This is particularly common in people of East Asian descent, who are more likely to carry a genetic variant that makes the second enzyme less efficient.
When someone drinks heavily, the liver recruits a backup system that only activates at high alcohol concentrations. This secondary pathway generates harmful byproducts called free radicals, which can damage liver cells over time. It’s one reason chronic heavy drinking leads to liver disease even when the person feels “used to” the amount they consume.
Ethanol vs. Other Types of Alcohol
Ethanol is the only alcohol that is safe to drink, and even that comes with risks. The word “alcohol” in chemistry refers to a whole family of compounds sharing that same hydroxyl group, and several common members of this family are extremely dangerous.
Methanol (CH3OH), sometimes called wood alcohol, is found in certain industrial solvents, antifreeze, and fuel. It looks and smells similar to ethanol, but your liver converts it into formaldehyde and then formic acid, both of which are far more toxic than ethanol’s byproducts. Formic acid attacks the optic nerve and specific structures deep in the brain. A potentially lethal dose of pure methanol is roughly 1 to 2 milliliters per kilogram of body weight, meaning just a few ounces could kill an average adult. Even non-fatal exposure can cause permanent blindness.
Isopropyl alcohol, the familiar “rubbing alcohol,” is another toxic cousin. It’s widely used as a disinfectant and cleaning agent but causes severe internal damage if swallowed. The packaging warnings on these products exist for good reason: the human body cannot safely metabolize these compounds.
Interestingly, one treatment for methanol poisoning is ethanol itself. Because both compounds are processed by the same liver enzyme, and ethanol has a stronger affinity for it, giving ethanol can effectively block methanol from being converted into its most dangerous byproducts, buying time for medical treatment.
Why Alcohol Affects People Differently
Two people drinking the same number of beers can feel very different effects. Body composition plays a major role: ethanol distributes through body water, and people with a higher percentage of body fat (and therefore less body water) reach higher blood alcohol levels from the same amount. This is one reason women, who on average carry proportionally more body fat than men, tend to reach higher BAC levels drink for drink, even at the same body weight.
Food slows alcohol absorption significantly. Drinking on an empty stomach allows ethanol to pass quickly into the small intestine, where it enters the bloodstream rapidly. A full meal, especially one with fat and protein, can slow absorption enough to noticeably blunt the peak BAC from the same number of drinks.
Tolerance is real but misleading. Regular drinkers develop neurological adaptations that make them feel less impaired at a given BAC, but their liver isn’t necessarily clearing the alcohol any faster. This means a tolerant drinker can feel functional at a BAC that would be genuinely dangerous, and the long-term organ damage accumulates regardless of whether they “feel drunk.”