Your blood alcohol level rises whenever alcohol enters your bloodstream faster than your liver can break it down. The average person’s liver clears roughly 7 grams of alcohol per hour, which is about one standard drink. Anything that speeds up absorption or slows down metabolism tips the balance toward a higher blood alcohol concentration (BAC). Several factors control that balance, from what you ate for dinner to your body composition and genetics.
How Alcohol Gets Into Your Blood
When you take a drink, the alcohol doesn’t need to be digested. It passes directly through the walls of your stomach and small intestine into your bloodstream. On an empty stomach, only about 10% of alcohol is absorbed in the stomach itself. The rest moves quickly into the small intestine, which has a much larger surface area and absorbs alcohol rapidly. This is the single biggest reason drinking on an empty stomach hits harder and faster: the alcohol rushes straight through to the intestine with almost nothing to slow it down.
When you eat before or while drinking, food triggers the valve at the bottom of your stomach to close partially, holding everything in the stomach longer. This slows the flow of alcohol into the small intestine. In one study of healthy adults, nearly 40% of ingested alcohol was absorbed through the stomach wall during the first hour after a meal, and the small intestine’s contribution dropped to a minor role. The practical result is a lower, more gradual peak BAC compared to drinking the same amount on an empty stomach.
Body Composition and Weight
Alcohol dissolves in water, not fat. Once it enters your bloodstream, it distributes throughout your body’s water content. A person with more muscle tissue (which holds a lot of water) has a larger “tank” to dilute the alcohol, resulting in a lower BAC. A person with a higher percentage of body fat has less water available, so the same amount of alcohol concentrates into a smaller volume and produces a higher reading. This is true even when two people weigh exactly the same.
This is also one reason why simply using body weight to predict BAC is unreliable. Two people at 160 pounds can have very different BAC levels after the same number of drinks if one carries significantly more body fat than the other.
Biological Sex
Women generally reach a higher BAC than men after consuming the same amount of alcohol, even after adjusting for body weight. Two things drive this difference. First, women tend to carry a higher proportion of body fat and less total body water, which concentrates alcohol in the blood. Second, women produce less of the stomach enzyme that breaks down alcohol before it ever reaches the bloodstream. A landmark study published in the New England Journal of Medicine found that women’s stomach enzyme activity was only 59% of men’s, and their “first-pass metabolism,” the amount of alcohol neutralized before it hits the bloodstream, was just 23% of men’s.
The gap widens further in people with alcohol use disorders. In men who drank heavily, the stomach enzyme activity dropped to about half of that in non-drinking men. In women who drank heavily, stomach enzyme activity fell even lower, and first-pass metabolism was essentially eliminated, meaning nearly all consumed alcohol entered the bloodstream intact.
Drinking Speed and Carbonation
The faster you drink, the faster alcohol accumulates in your blood. Your liver processes alcohol at a relatively fixed rate, so pounding two drinks in 20 minutes delivers far more alcohol than your liver can handle in that window. The excess circulates in your blood, pushing BAC up sharply.
Carbonation also plays a role. Mixing alcohol with a carbonated beverage (soda, tonic water, sparkling wine) can speed up absorption compared to the same drink made with a still mixer. In a controlled study, two-thirds of participants absorbed alcohol significantly faster when it was mixed with a carbonated drink. The carbonation appears to push alcohol through the stomach lining and into the intestine more quickly. This is part of why champagne or a rum and Coke may feel like it hits faster than sipping the same amount of alcohol in a non-bubbly form.
Genetics and Enzyme Variation
Your genes determine how efficiently your body breaks down alcohol, and there’s a 3- to 4-fold difference in alcohol elimination rates across the population. The biggest genetic factor involves the enzymes your liver and stomach use to process ethanol.
One well-studied example is a variant of the enzyme responsible for clearing acetaldehyde, the toxic byproduct your body creates when it breaks down alcohol. Roughly 8% of the world’s population (and up to 40% of people of East Asian descent) carry a mutation that makes this enzyme far less effective. In these individuals, acetaldehyde builds up rapidly, causing facial flushing, nausea, and a rapid heartbeat. While this variant doesn’t necessarily raise BAC itself, it dramatically alters how the body processes and reacts to alcohol, and it changes how long toxic byproducts linger in the system.
Age
As you get older, the same number of drinks produces a higher BAC than it did when you were younger. The main reason is body composition: aging reduces lean muscle mass and total body water while increasing body fat percentage. Research on ethanol metabolism across age groups found a clear correlation between age and peak blood alcohol concentration, driven primarily by a shrinking volume of distribution. In practical terms, a 65-year-old who drinks the same amount as they did at 35 will likely reach a noticeably higher BAC, even if their weight hasn’t changed.
Reduced liver blood flow in older adults can also slow the rate at which the liver encounters and processes circulating alcohol, though body composition remains the dominant factor.
Medications
Certain common medications interfere with the enzymes that break down alcohol, effectively raising your BAC by reducing first-pass metabolism. Aspirin has been shown to significantly reduce the amount of alcohol your stomach neutralizes before it reaches the bloodstream. Common pain relievers like acetaminophen and some heartburn medications also bind to the same enzymes involved in alcohol metabolism, potentially slowing the breakdown of both alcohol and its toxic byproduct acetaldehyde.
This doesn’t mean every medication will spike your BAC, but if you take pain relievers, antacids, or other drugs before drinking, the interaction can meaningfully raise peak blood alcohol levels beyond what you’d expect from the drinks alone.
Drink Strength and Volume
A U.S. standard drink contains 14 grams (0.6 ounces) of pure alcohol. That’s 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of distilled spirits. But many real-world drinks exceed this. A craft IPA at 8% ABV in a 16-ounce glass contains roughly twice the alcohol of a standard drink. A generous pour of wine at a restaurant can easily be 7 or 8 ounces. Mixed cocktails at bars often contain two or more shots.
People frequently underestimate how much alcohol they’re actually consuming, which is one of the most straightforward reasons BAC climbs higher than expected. Counting “number of drinks” only works if each drink is actually one standard serving, and in practice, that’s rarely the case.
How These Factors Stack
These factors don’t operate in isolation. A smaller woman drinking carbonated cocktails on an empty stomach after taking aspirin will reach a dramatically higher BAC than a larger man eating a full meal and sipping a still drink slowly. The combination of faster absorption, less body water for dilution, and reduced enzyme activity can multiply the effect of each individual factor. Understanding which variables apply to you is the most reliable way to anticipate how alcohol will affect your blood alcohol level on any given occasion.