Your liver does the heavy lifting when it comes to alcohol, breaking down more than 90% of what you drink through a two-step enzymatic process. The remaining 2 to 5% leaves your body unchanged through your breath, urine, and sweat. That small fraction is what makes breathalyzer tests possible, but the real story of alcohol metabolism happens inside liver cells, where enzymes convert ethanol into increasingly harmless compounds until it’s nothing but water and carbon dioxide.
The Two-Step Breakdown in Your Liver
Alcohol metabolism follows a straightforward chain reaction. First, an enzyme called alcohol dehydrogenase (ADH) converts ethanol into acetaldehyde, a toxic compound classified as a known carcinogen. This is the dangerous middle step. Fortunately, acetaldehyde is short-lived in most people because a second enzyme, aldehyde dehydrogenase (ALDH), quickly converts it into acetate, a much less harmful substance. Acetate then gets broken down further into water and carbon dioxide, which your body eliminates easily.
Think of it as a disassembly line: ethanol enters, gets stripped into a toxic intermediate, then that intermediate is rapidly neutralized. The speed of that second step matters enormously. When acetaldehyde lingers in your system, it damages cells and DNA. How quickly your body clears it depends largely on your genetics and how much you drink.
What Happens Before Alcohol Reaches Your Liver
Alcohol absorption starts the moment you take a drink, and it begins well before your liver gets involved. Ethanol absorbs slowly through the stomach lining but rapidly through the small intestine. This means the rate your stomach empties its contents into the small intestine is one of the biggest factors controlling how fast alcohol hits your bloodstream.
Eating before or while drinking slows gastric emptying, which delays absorption and lowers your peak blood alcohol concentration. This is why drinking on an empty stomach produces a noticeably faster, stronger effect. The type of food matters less than simply having something in your stomach to slow the process down.
Your stomach also performs a small but meaningful first pass at metabolism before alcohol even reaches the liver. Enzymes in the stomach lining break down a portion of alcohol on contact. This “first-pass metabolism” varies significantly between individuals. Research published in the New England Journal of Medicine found that women had only about 23% of the first-pass metabolism seen in men, partly because their gastric ADH activity was 59% lower. In people with alcohol use disorder, this gastric metabolism drops further, and in women with chronic heavy drinking, it was virtually abolished.
How Fast Your Body Clears Alcohol
The average person clears alcohol from the bloodstream at roughly 20 mg/dL per hour. In practical terms, that means processing one standard U.S. drink (14 grams of pure alcohol, equivalent to a 12-ounce beer, 5-ounce glass of wine, or 1.5-ounce shot of spirits) takes roughly one to one and a half hours. But the individual variation is wide. Studies of emergency department patients found a standard deviation of nearly 7 mg/dL per hour, meaning some people clear alcohol at 13 mg/dL per hour while others manage 27 or more.
This process is largely zero-order, meaning your liver works at a fixed maximum rate regardless of how much alcohol is in your system. You can’t speed it up by drinking water, coffee, or exercising. Once your liver’s enzymes are fully occupied, any additional alcohol simply waits in your bloodstream.
The Backup System for Heavy Drinking
Your liver has a secondary processing pathway that normally handles only a small share of alcohol metabolism. This system, centered on a liver enzyme called CYP2E1, becomes much more active during chronic or heavy drinking. After sustained alcohol exposure, CYP2E1 levels increase two- to threefold, effectively expanding the liver’s capacity to process ethanol.
This sounds like it would be helpful, but it comes with a serious downside. CYP2E1 generates significantly more harmful byproducts called reactive oxygen species, which cause oxidative stress and damage liver cells. This is one of the key mechanisms behind alcohol-related liver disease. The enzyme also interacts with certain medications, breaking them down faster or converting them into toxic forms. This is why chronic heavy drinkers can have unexpected reactions to common drugs like acetaminophen.
Why the Same Drink Hits People Differently
Several biological factors create dramatic differences in how two people experience the same amount of alcohol.
Body composition: Alcohol disperses in water, not fat. Women generally carry proportionally more body fat and less body water than men of the same weight, so they reach higher peak blood alcohol concentrations from equivalent doses. In one study, the gender difference in blood alcohol levels disappeared entirely when researchers dosed based on total body water rather than body weight.
Enzyme activity in the stomach: As noted above, gastric ADH activity is 70 to 80% higher in men who don’t drink heavily compared to women. This means more alcohol gets neutralized in a man’s stomach before it ever enters the bloodstream. There’s also evidence that the male hormone dihydrotestosterone may slow liver ADH activity, but women compensate in a different way: they eliminate more alcohol per volume of blood per hour than men do, likely due to differences in liver-to-body-weight ratio.
Genetics: The most dramatic example is the ALDH2*2 genetic variant, carried by roughly 540 million people worldwide, predominantly those of East Asian descent. This variant produces an inactive or less active version of the enzyme responsible for clearing acetaldehyde (the toxic intermediate). People who carry it accumulate acetaldehyde after drinking, which triggers the “alcohol flush response”: facial redness, rapid heartbeat, nausea, and general discomfort. Beyond being unpleasant, this acetaldehyde buildup carries real health risks, as chronic exposure to elevated acetaldehyde increases cancer risk even at moderate drinking levels.
Where the Energy Goes
Alcohol contains about 7 calories per gram, nearly as much as fat. Once your liver converts ethanol to acetate, that acetate enters the bloodstream and gets picked up by cells throughout your body. Inside those cells, acetate is converted into acetyl-CoA, a molecule that feeds into the same energy-production cycle your body uses for carbohydrates and fats.
The catch is that your body prioritizes alcohol metabolism over everything else. While your liver is busy processing ethanol, it largely pauses its normal work of burning fat and regulating blood sugar. This is why alcohol promotes fat storage even though the alcohol itself is being “burned” for energy. Your liver essentially shifts all its resources to clearing a substance it treats as a toxin, and other metabolic tasks get put on hold until the job is done.
The 2 to 5% That Escapes Unchanged
A small fraction of the alcohol you drink never gets metabolized at all. It passes through your body and exits unchanged through three routes: your lungs exhale it, your kidneys filter it into urine, and a trace amount leaves through sweat. This fraction is tiny compared to what your liver handles, but it’s the entire basis for breath and urine alcohol testing. The concentration of alcohol in your exhaled breath maintains a predictable ratio to the concentration in your blood, which is what allows a breathalyzer to estimate blood alcohol levels from a simple exhale.