Alcohol metabolism is the process your body uses to break down ethanol into substances it can safely eliminate. Most of this work happens in the liver, where enzymes convert alcohol first into a toxic intermediate compound, then into harmless byproducts: water and carbon dioxide. The entire process unfolds in stages, each with its own consequences for your health.
The Two-Step Breakdown
Your liver handles the vast majority of alcohol processing through a two-enzyme system. In the first step, an enzyme called alcohol dehydrogenase (ADH) converts ethanol into acetaldehyde. This is a critical moment, because acetaldehyde is highly toxic and a known carcinogen. Fortunately, it doesn’t stick around long. A second enzyme, aldehyde dehydrogenase (ALDH), quickly converts acetaldehyde into acetate, a much less harmful substance. Acetate then leaves the liver and is broken down into water and carbon dioxide in other tissues throughout the body.
This sequence is constant and predictable. The average person clears alcohol from the bloodstream at a rate of about 20 mg/dL per hour, which roughly translates to processing one standard drink every 60 to 90 minutes. You cannot speed this up with coffee, food, or cold showers. The liver works at a fixed pace, and any alcohol beyond what it can handle at that moment continues circulating in your blood, which is what produces intoxication.
What Happens When You Drink Heavily
The two-enzyme pathway handles most alcohol processing, but it has limits. When alcohol intake is high, particularly in people who drink regularly, a backup system kicks in. This secondary pathway, called the microsomal ethanol oxidizing system (MEOS), also produces acetaldehyde but normally accounts for less than 10 percent of the liver’s capacity to process ethanol.
Here’s what changes with chronic drinking: the MEOS pathway becomes more active over time. The body ramps up production of a specific enzyme (CYP2E1) that drives this system, effectively increasing the liver’s total capacity to process alcohol. This is one reason heavy drinkers develop metabolic tolerance, meaning their bodies can clear alcohol faster than someone who rarely drinks. But increased CYP2E1 activity comes with a cost. This enzyme generates harmful molecules called free radicals during the process, which contribute to liver inflammation and damage over time.
Why Acetaldehyde Is So Dangerous
Acetaldehyde, the toxic intermediate your body creates during the first step of metabolism, is more than just an unpleasant byproduct. It directly damages DNA in multiple ways: it forms abnormal attachments to DNA molecules, causes strand breaks, triggers point mutations, and creates cross-links between DNA strands. These kinds of genetic disruptions are the hallmarks of cancer development. Acetaldehyde also blocks normal DNA replication, forcing cells to rely on error-prone repair processes that introduce even more mutations.
In most people, ALDH clears acetaldehyde quickly enough that exposure is brief. But anything that slows ALDH activity, whether genetics, heavy drinking, or both, extends the window during which acetaldehyde is causing damage to cells.
The Metabolic Ripple Effect
Breaking down alcohol doesn’t just produce toxic intermediates. It also disrupts normal liver chemistry in a way that affects how your body handles fats and sugar. Both steps of alcohol metabolism generate large amounts of a molecule called NADH, which shifts a critical chemical balance inside liver cells. This imbalance has several consequences at once: it slows the liver’s ability to burn fat for energy, ramps up the production of new fat, and suppresses the liver’s ability to produce glucose.
This is why heavy drinking leads to fat accumulation in the liver, a condition known as fatty liver or hepatic steatosis. It also explains why alcohol can cause dangerously low blood sugar, especially in people who haven’t eaten. The liver, normally responsible for maintaining blood sugar between meals, is essentially too busy processing alcohol to do its regular job.
Why Women Process Alcohol Differently
Before alcohol even reaches the liver, some of it is broken down by ADH enzymes in the stomach lining. This “first pass” metabolism reduces the amount of alcohol that enters the bloodstream. But stomach ADH activity is not equal across all people.
Women under 50 have significantly lower stomach ADH activity compared to men of the same age. In one study, the enzyme activity in young women’s stomachs was roughly 30 percent lower than in age-matched men. This means more alcohol passes through the stomach intact and enters the bloodstream, producing higher blood alcohol levels from the same number of drinks. Interestingly, this gap narrows after age 50, because stomach ADH activity in men declines with age while it remains relatively stable in women. People with alcohol use disorder also show reduced stomach ADH activity regardless of sex.
Genetics and the Flush Response
Not everyone’s metabolic enzymes work the same way. Genetic variations in both ADH and ALDH can dramatically change the experience of drinking. The most well-known example involves a variant of the ALDH2 gene that is more common among people of East Asian descent, though it can occur in any population.
People who carry this variant have a less efficient version of the enzyme that clears acetaldehyde. The result is a buildup of acetaldehyde after drinking, which triggers the release of histamine and produces what’s known as the alcohol flush reaction: facial redness, hives, nausea, low blood pressure, and sometimes worsening of asthma or migraine. This isn’t just uncomfortable. Because acetaldehyde is a carcinogen, people with this variant who drink regularly face a higher risk of certain cancers, particularly of the esophagus and upper digestive tract.
Other genetic variants speed up the first step of metabolism (converting alcohol to acetaldehyde) without speeding up the second step, creating the same bottleneck of toxic acetaldehyde.
Where Else Metabolism Happens
The liver does the heavy lifting, but it isn’t the only organ that processes alcohol. The CYP2E1 enzyme that drives the backup pathway is found in the brain, heart, lungs, and certain immune cells. In the brain, where the primary ADH enzyme is present in very low amounts, CYP2E1 plays a more prominent role in local alcohol metabolism. This means the brain is directly exposed to the harmful byproducts of alcohol processing, not just the effects of circulating ethanol.
This distribution of metabolic activity across organs helps explain why chronic alcohol use damages so many different parts of the body, not just the liver.
Alcohol and Medication Conflicts
The CYP2E1 enzyme doesn’t just process alcohol. It also metabolizes certain medications, which creates the potential for dangerous interactions. The most significant example is acetaminophen (Tylenol). CYP2E1 converts a portion of acetaminophen into a byproduct that is toxic to the liver. In people who drink heavily, CYP2E1 activity is elevated, so when they take acetaminophen, more of it gets converted into this harmful compound. This is why combining regular heavy drinking with even normal doses of acetaminophen can cause serious liver damage.
The timing matters too. The risk is highest when acetaminophen is taken after a period of heavy drinking rather than at the same time, because alcohol and acetaminophen compete for the same enzyme. Once alcohol is cleared and CYP2E1 is available, it processes acetaminophen aggressively, producing more of the toxic byproduct than it normally would.