Your body breaks down alcohol in a two-step chemical process, primarily in the liver. First, an enzyme converts ethanol into a toxic intermediate called acetaldehyde. Then, a second enzyme quickly converts that acetaldehyde into acetate, which is eventually broken down into water and carbon dioxide and eliminated. The whole system processes alcohol at a roughly fixed rate of about one standard drink per hour, regardless of how much you’ve consumed.
The Two-Step Breakdown in the Liver
About 90 to 95 percent of the alcohol you drink is processed in the liver. The first enzyme involved, alcohol dehydrogenase (ADH), strips electrons from ethanol and converts it into acetaldehyde. This is the critical step, because acetaldehyde is highly toxic and a known carcinogen. Even brief exposure to it causes damage to cells and DNA.
Fortunately, acetaldehyde doesn’t stick around for long. A second enzyme, aldehyde dehydrogenase (ALDH), rapidly converts it into acetate, a much less harmful substance. Acetate then leaves the liver, enters the bloodstream, and is eventually broken down into water and carbon dioxide that your body can easily get rid of through breathing, sweat, and urine.
Both of these reactions require a helper molecule called NAD+, which gets converted to NADH in the process. This matters because it shifts the chemical balance inside your liver cells. The buildup of NADH interferes with other important liver functions, including the production of new glucose and the normal processing of fats. This is one reason heavy drinking leads to fatty liver disease, low blood sugar, and a buildup of lactic acid in the blood.
What Happens at Higher Drinking Levels
When you drink moderately, ADH handles most of the work. But when blood alcohol levels rise, a backup system kicks in. A second pathway centered on an enzyme called CYP2E1 starts picking up more of the load. This enzyme sits in a different part of the liver cell and becomes especially active at elevated alcohol concentrations.
CYP2E1 also becomes more abundant in people who drink regularly. Chronic alcohol use causes the body to produce more of this enzyme, which is one reason heavy drinkers can temporarily “tolerate” more alcohol. But this comes at a cost: CYP2E1 generates harmful molecules called free radicals as a byproduct of breaking down ethanol, contributing to liver inflammation and damage over time. Nicotine also increases CYP2E1 activity, which is one reason people who both smoke and drink heavily face compounded health risks.
How Alcohol Is Processed in the Brain
The liver does the heavy lifting, but your brain also metabolizes small amounts of alcohol locally. The dominant enzyme there isn’t ADH. Instead, an enzyme called catalase handles roughly 60 percent of ethanol breakdown in the brain, with CYP2E1 contributing about 20 percent. The remaining 20 percent involves pathways that aren’t fully understood.
This local metabolism matters because it produces acetaldehyde directly inside brain tissue. Research in rodents shows that blocking catalase activity nearly cuts brain acetaldehyde production in half. Some of the behavioral effects of drinking, including certain rewarding sensations, may actually be driven by this locally produced acetaldehyde rather than by ethanol itself.
Why Your Body Can Only Process So Much Per Hour
Alcohol metabolism follows what pharmacologists call zero-order kinetics. In plain terms, that means your body clears alcohol at a fixed rate, not a rate proportional to how much is in your system. Most drugs are cleared faster when concentrations are high and slower when they drop. Alcohol doesn’t work that way. The enzymes involved become saturated quickly, and from that point on, your liver works at a constant speed.
The average clearance rate is about 20 mg/dL per hour, which works out to roughly one standard drink every 60 to 90 minutes. But individual variation is substantial. In one study of emergency department patients, the standard deviation was nearly 7 mg/dL per hour, meaning some people cleared alcohol at 8 mg/dL per hour and others at 32. Only about 83 percent of people fell within that range. Drinking coffee, taking a cold shower, or exercising does nothing to speed up this fixed enzymatic process.
Why Men and Women Process Alcohol Differently
Men generally have higher levels of ADH in the stomach lining than women do. This means some alcohol gets broken down before it even reaches the bloodstream, a process called first-pass metabolism. With less stomach ADH, women absorb a higher percentage of the alcohol they drink, leading to higher peak blood alcohol concentrations from the same dose.
Body composition amplifies this difference. Alcohol dissolves in water, not fat. Women typically have a higher ratio of body fat to water than men of the same weight, so the same amount of alcohol gets distributed into a smaller volume of water, producing a higher concentration. Two people who weigh exactly the same can reach very different blood alcohol levels from an identical number of drinks because of differences in body water content alone.
Genetic Differences That Change the Process
Nearly one-third of people of East Asian descent carry a variant of the ALDH2 gene called ALDH2*2. This variant produces a less active version of the enzyme responsible for clearing acetaldehyde. The result is that acetaldehyde builds up to much higher levels after drinking, causing facial flushing, nausea, rapid heartbeat, and general discomfort. This reaction is sometimes called “Asian flush” or “alcohol flush reaction.”
The ALDH2*2 variant acts as a natural deterrent to drinking, and many people who carry it avoid alcohol entirely. But those who drink despite carrying the variant face serious long-term consequences. The sustained acetaldehyde exposure increases the risk of cancers in multiple organs and accelerates age-related neuronal damage. Research links ALDH2 deficiency to a higher risk of late-onset Alzheimer’s disease and alcohol-related nerve damage compared to people with fully functional ALDH2.
How Food Changes Alcohol Absorption
Eating before or while you drink doesn’t change how fast your liver metabolizes alcohol, but it significantly changes how quickly alcohol reaches your bloodstream in the first place. Food in the stomach slows gastric emptying, the rate at which stomach contents pass into the small intestine, where most alcohol absorption occurs. The result is a lower peak blood alcohol concentration and a longer time to reach that peak.
The type of food matters less than simply having something in your stomach, though meals with fat and protein tend to slow gastric emptying more than simple carbohydrates. Drinking on an empty stomach allows alcohol to pass rapidly into the small intestine, where it’s absorbed almost immediately. This is why the same number of drinks can feel dramatically different depending on whether you’ve eaten.