Blood alcohol concentration, or BAC, is shaped by a surprisingly long list of factors beyond just how many drinks you’ve had. Your body size, biological sex, genetics, age, whether you’ve eaten, and even whether your drink is carbonated all play a role in how high your BAC climbs and how long it stays elevated. Understanding these factors helps explain why two people can drink the same amount and end up with very different levels of impairment.
Body Size and Composition
The single biggest physical factor in BAC is how much water is in your body. Alcohol dissolves in water, so a person with more total body water will dilute the same amount of alcohol across a larger volume, resulting in a lower BAC. This is why a larger person generally reaches a lower BAC than a smaller person after the same number of drinks. But total body weight alone doesn’t tell the full story. Body composition matters too: muscle tissue holds more water than fat tissue, so two people at the same weight can have different BACs if one carries more body fat.
Biological Sex
Women typically reach higher BACs than men of similar body weight after drinking the same amount. The primary reason is that women generally have a lower proportion of total body water, which means there’s less fluid to dilute the alcohol. Women also tend to have differences in stomach enzyme activity that affect how much alcohol is broken down before it ever reaches the bloodstream. These are not small differences. They’re significant enough that standard BAC charts use separate columns for men and women.
Food in Your Stomach
Eating before or while you drink is one of the most practical ways to influence your BAC. Food slows the rate at which alcohol passes from your stomach into your small intestine, where most absorption happens. In one study, participants drinking on a full stomach had notably lower peak BAC levels compared to drinking on an empty stomach, even though the total time for alcohol to fully leave their system was about the same (roughly five hours in both cases). The peak BAC level still arrived at around 41 minutes in both conditions, but the height of that peak was meaningfully lower with food present. A meal high in fat or protein is especially effective at slowing absorption.
Carbonation and Drink Type
Carbonated drinks speed up alcohol absorption. Researchers comparing carbonated mixers to still (flat) mixers found that the carbonated version produced an absorption rate roughly four times faster. This is why champagne, sparkling wine, or spirits mixed with tonic or soda can hit you harder and faster than the same amount of alcohol in a non-carbonated form. The carbonation appears to push alcohol through the stomach lining and into the small intestine more quickly.
The alcohol concentration of a beverage also matters. Drinks in the moderate range (around 20 to 30 percent alcohol by volume) tend to be absorbed faster than very dilute drinks like beer or very concentrated ones like straight spirits, which can irritate the stomach lining and slow gastric emptying.
How Fast You Drink
Your liver can only process alcohol at a relatively fixed rate. The average person clears about 20 mg/dL per hour, though individual variation is wide. Some people clear as little as 8 mg/dL per hour, while others clear up to 32 mg/dL per hour. When you drink faster than your liver can keep up, the excess alcohol accumulates in your bloodstream and your BAC rises. Spacing drinks out gives your body time to metabolize each one before the next arrives. After your last drink, BAC typically peaks somewhere between 60 and 90 minutes later, then begins to decline.
Genetics
Your genes determine which versions of alcohol-processing enzymes your body produces, and these variations can dramatically affect how quickly you break down alcohol. Two key enzyme families do the heavy lifting: one converts alcohol into a toxic intermediate compound called acetaldehyde, and the other breaks acetaldehyde down into harmless byproducts.
Some people carry gene variants that speed up the first step, producing acetaldehyde faster than normal. Others carry variants that slow the second step, leaving acetaldehyde to build up. That buildup causes flushing, nausea, and rapid heartbeat. The most well-known example is the ALDH2*2 variant, common in people of East Asian descent. People with one copy of this variant retain only 12 to 20 percent of normal enzyme activity for clearing acetaldehyde. People with two copies have essentially no activity at all, leading to severe reactions even after small amounts of alcohol.
These genetic differences don’t just change how you feel. They change the effective concentration of alcohol and its byproducts in your blood, influencing both your BAC trajectory and your subjective experience of being drunk.
Age
As you get older, the same amount of alcohol produces a higher BAC. After age 65, lean body mass and total body water both decline, meaning there’s less volume to dilute alcohol. Metabolism also slows with age, so alcohol stays in your system longer. The practical result is that a drink at 70 affects you more than the same drink did at 40, even if your weight hasn’t changed much.
Medications
Certain medications can raise your BAC by interfering with the enzymes that break down alcohol in your stomach. Common heartburn medications (H2 blockers like cimetidine and ranitidine) and even aspirin can block stomach enzyme activity, meaning more alcohol passes into your bloodstream intact. Other drugs that speed up gastric emptying, such as certain antibiotics and motility agents, can also push alcohol into the small intestine faster, accelerating absorption. If you take any regular medication, it’s worth knowing whether it interacts with alcohol.
Legal BAC Limits Vary Widely
In the United States, the legal limit for driving is 0.08 percent BAC for most drivers. England and Wales set their limit at 0.08 as well, while Scotland uses 0.05. Most of continental Europe sets limits between 0.02 and 0.05, and several countries, including the Czech Republic, Hungary, Romania, and Slovakia, enforce a strict zero-tolerance policy. Many jurisdictions also impose lower limits for commercial drivers and new drivers. These limits exist because even modest BAC levels impair reaction time, judgment, and coordination in ways that are difficult to self-assess.
One of the most important things to understand about BAC is that you can’t reliably estimate your own. The factors above interact in complex ways, and individual variation in metabolism alone spans a fourfold range. Two people at the same table, drinking the same drinks, can be at very different BAC levels and very different levels of impairment.