Hangovers are your body’s reaction to processing a toxic substance. When you drink alcohol, your liver breaks it down into a compound called acetaldehyde, which is significantly more toxic than alcohol itself. That breakdown process, combined with dehydration, stomach irritation, and an inflammatory immune response, produces the headache, nausea, fatigue, and general misery you feel the next morning.
No single mechanism explains the entire hangover. It’s several things going wrong at once, each responsible for different symptoms. Here’s what’s actually happening inside your body.
How Your Body Breaks Down Alcohol
Your liver processes alcohol in two steps. First, an enzyme converts ethanol into acetaldehyde. Second, another enzyme converts acetaldehyde into acetate, a relatively harmless substance your body can use for energy. The problem is that the first step often outpaces the second, especially when you’ve been drinking heavily. Acetaldehyde builds up in your system faster than your body can clear it, and it’s toxic to cells and tissues throughout your body.
Both steps depend on the same helper molecule (a coenzyme called NAD+), and your supply of it is limited. When your liver is busy processing large amounts of alcohol, NAD+ gets used up quickly, slowing down other metabolic processes that depend on it. This is one reason you can feel so depleted the next day: your liver has essentially been running a chemical marathon, burning through resources that other systems need.
People who drink heavily or chronically also activate a backup processing pathway that generates additional acetaldehyde. This secondary system kicks in when the primary one can’t keep up, but it produces the same toxic intermediate. So the more you drink, the more acetaldehyde accumulates, and the worse you tend to feel.
Why You Get So Dehydrated
Alcohol suppresses a hormone that tells your kidneys to hold onto water. Without that signal, your kidneys let far more fluid pass through than they normally would. One older estimate found that for every 10 grams of alcohol consumed (roughly the amount in a standard drink), your body produces an extra 100 milliliters of urine beyond what it normally would. Over the course of an evening, that adds up to a significant amount of lost fluid.
This diuretic effect doesn’t just strip water from your body. You also lose electrolytes, the minerals that help regulate nerve signaling, muscle function, and hydration balance. That’s why a hangover headache often feels different from a regular headache: it’s partly driven by the brain’s surrounding membranes pulling on their attachments as fluid levels drop. The thirst, dizziness, and lightheadedness are all downstream effects of this fluid and electrolyte loss.
What Causes the Nausea and Stomach Pain
Alcohol directly irritates the lining of your stomach. It also triggers your stomach to produce more acid than usual. Together, these two effects explain why nausea, belly pain, and vomiting are among the most common hangover symptoms. The irritation can persist well after you’ve stopped drinking because your stomach lining doesn’t recover instantly, and the excess acid continues to cause discomfort.
This is also why reaching for aspirin or ibuprofen the morning after can backfire. Both of those pain relievers stimulate additional acid production, which can make an already irritated stomach feel worse.
Your Immune System Gets Involved
Hangovers aren’t just a chemical cleanup problem. Your immune system responds to a night of heavy drinking much the way it responds to an infection: by releasing inflammatory signaling molecules called cytokines. Research has measured changes in several of these molecules during hangovers, and while not every inflammatory marker spikes (some studies found no change in commonly studied ones like IL-6 and TNF-alpha), the overall pattern points to a low-grade inflammatory state.
This immune activation likely explains symptoms that seem disproportionate to simple dehydration or stomach upset: the brain fog, the fatigue, the aching muscles, the difficulty concentrating. These overlap heavily with the symptoms you experience when you’re fighting off a cold, which makes sense because the underlying mechanism is similar. Your body is mounting an inflammatory response, and that response makes you feel lousy whether the trigger is a virus or a night of heavy drinking.
Why Dark Liquors Make It Worse
Not all alcoholic drinks produce equal hangovers. Darker spirits like bourbon and whiskey contain higher levels of compounds called congeners, which are byproducts of fermentation and aging. These include small amounts of methanol, tannins, and other chemicals that your body has to process alongside the ethanol itself.
Research comparing bourbon drinkers to vodka drinkers found that bourbon produced noticeably more severe hangover symptoms, even when the total amount of alcohol consumed was the same. Vodka contains very few congeners. This doesn’t mean clear spirits won’t give you a hangover (they absolutely will if you drink enough), but the additional chemical load from congeners adds an extra layer of misery. The general pattern holds across drink types: darker alcohols like red wine, brandy, and dark rum tend to produce worse hangovers than their lighter counterparts.
Why Some People Get Worse Hangovers
Genetics play a significant role. The enzymes responsible for breaking down alcohol and acetaldehyde vary in efficiency from person to person. Some people, particularly those of East Asian descent, carry a genetic variant that makes their acetaldehyde-clearing enzyme work much more slowly. This leads to a faster buildup of acetaldehyde after drinking, producing flushing, nausea, and more severe hangover symptoms.
Body composition matters too. People with more body water (generally those who are larger or have more muscle mass) dilute alcohol more effectively, which slows the rate of acetaldehyde accumulation. Age is another factor: as you get older, your liver processes alcohol less efficiently, and your body holds less water overall. Both changes make hangovers progressively worse with age, which is why a night of drinking at 40 feels very different from the same night at 22.
Why Hangovers Peak After You’ve Stopped Drinking
One of the counterintuitive things about hangovers is that they tend to be worst when your blood alcohol level has dropped back to zero or near zero. This timing makes sense when you consider the mechanisms involved. Acetaldehyde accumulation, dehydration, and immune activation are all processes that lag behind the drinking itself. Your body is still dealing with the aftermath hours after your last drink.
Sleep disruption compounds this. Alcohol initially makes you drowsy, but as your body metabolizes it overnight, the withdrawal effect fragments your sleep during the second half of the night. You spend less time in the deep, restorative sleep stages. So even if you slept for eight hours, you wake up feeling unrested, which amplifies every other hangover symptom.
The combination of all these mechanisms, none of which would be unbearable on its own, is what makes a hangover such a miserable experience. Dehydration alone causes a headache. Stomach irritation alone causes nausea. Immune activation alone causes fatigue. Stack them together, add fragmented sleep, and you get the full syndrome.