Alcohol causes a coma by suppressing the brain so deeply that it can no longer maintain consciousness, protect the airway, or regulate breathing. This typically happens at blood alcohol concentrations (BAC) above 0.30%, with levels above 0.40% carrying a serious risk of death. But direct brain suppression isn’t the only pathway. Alcohol can also trigger a coma indirectly through dangerously low blood sugar, disrupted electrolytes, or liver failure that allows toxins to build up in the brain.
How Alcohol Shuts Down the Brain
Alcohol is a central nervous system depressant. At moderate doses, it slows reaction time and impairs judgment. At very high doses, it effectively sedates the brain into unconsciousness. This happens through two simultaneous effects on brain chemistry.
First, alcohol amplifies the activity of your brain’s main “braking” system. Your neurons use a chemical signal called GABA to calm each other down, and alcohol makes GABA receptors far more responsive than normal. Second, alcohol blocks the receptors that your brain uses for its main “accelerator” signal, glutamate. With the brakes floored and the accelerator cut, brain activity drops dramatically. At high enough concentrations, this suppression reaches the brainstem, the primitive region that controls breathing, heart rate, and body temperature. When the brainstem stops functioning properly, breathing slows or stops entirely.
Blood Alcohol Levels That Cause a Coma
A BAC between 0.30% and 0.40% typically causes alcohol poisoning and loss of consciousness. Above 0.40%, you’re at risk of coma and death from respiratory arrest. For context, the legal driving limit in the U.S. is 0.08%, so a coma-level BAC is roughly four to five times that threshold.
How quickly someone reaches these levels depends on body weight, drinking speed, food in the stomach, tolerance, and whether other substances are involved. Binge drinking, especially consuming large amounts in a short window, is the most common route to a life-threatening BAC because the liver can only process about one standard drink per hour. Everything beyond that accumulates in the bloodstream.
Low Blood Sugar as a Hidden Trigger
Alcohol doesn’t just suppress the brain directly. It also interferes with the liver’s ability to produce glucose, your brain’s primary fuel. When you drink heavily, the chemical process of breaking down alcohol inside the liver shifts the balance of key molecules the liver needs to manufacture new glucose. Specifically, the reaction that metabolizes alcohol depletes a molecule called NAD+, which the liver requires to convert stored energy into blood sugar. Without enough NAD+, glucose production stalls.
If blood sugar drops low enough, the brain simply runs out of energy. This can cause confusion, seizures, loss of consciousness, and coma. The risk is highest in people who have been drinking heavily without eating, since their glycogen stores (the liver’s backup supply of glucose) are already depleted. This type of coma can occur even when BAC isn’t at the extreme levels typically associated with alcohol poisoning.
Electrolyte Disruption and Brain Swelling
Chronic heavy drinking and alcoholism can lead to dangerously low sodium levels, a condition called hyponatremia. Sodium helps regulate the balance of water inside and outside your cells. When sodium drops too low, water flows into brain cells to try to restore balance, causing them to swell. This swelling increases pressure inside the skull, which can damage brain tissue and lead to seizures, coma, or death.
Malnutrition and poor dietary habits common in people with alcohol use disorder make this worse. The body loses its ability to properly manage water and sodium, creating the conditions for sudden, dangerous shifts in brain fluid balance.
Liver Failure and Toxic Buildup
Years of heavy drinking can cause cirrhosis, where scar tissue gradually replaces healthy liver tissue. One of the liver’s critical jobs is filtering ammonia, a toxic byproduct of protein digestion, out of the blood. When the liver fails, ammonia accumulates in the bloodstream and crosses into the brain.
This condition, called hepatic encephalopathy, progresses through stages. Early on, it causes subtle changes in thinking and personality that might go unnoticed. As it worsens, it leads to obvious confusion, disorientation, and slurred speech. In its most severe form (classified as grade 4 on the clinical scale used by doctors), it causes full coma with complete unresponsiveness. Recent research has shown that the damage isn’t from ammonia alone. Systemic inflammation, common in people with cirrhosis, makes the brain more vulnerable to ammonia’s effects by increasing the permeability of the blood-brain barrier and triggering swelling in brain cells.
Choking on Vomit and Oxygen Loss
There’s another path to coma that doesn’t involve the brain’s chemistry at all. Heavily intoxicated people lose their gag reflex. If they vomit while unconscious, stomach contents can enter the lungs, blocking airflow. This is called aspiration, and it’s a well-documented danger in severe alcohol intoxication. One study of patients hospitalized for acute alcohol poisoning found that those who aspirated had significantly lower oxygen levels (averaging 90% compared to 96% in those who didn’t) and much lower consciousness scores on arrival.
Without adequate oxygen, brain cells begin dying within minutes. Even if breathing is restored, the resulting brain injury can leave someone in a prolonged coma or cause permanent damage.
What Happens in the Emergency Room
When someone arrives at an emergency department in an alcohol-induced coma, the immediate priority is keeping them breathing. Medical staff will position the airway, provide oxygen, and if necessary, use a breathing tube to prevent suffocation. They check blood sugar and correct it with intravenous glucose if it’s dangerously low. Blood tests look for electrolyte imbalances, liver function problems, and the presence of other substances that may have been consumed alongside alcohol.
There is no antidote for alcohol. The body has to metabolize it on its own, so emergency care is about keeping the person alive and stable while that happens. Clinicians use a scoring system called the Glasgow Coma Scale to track consciousness, where a score of 8 or below indicates a coma. Patients are monitored continuously because the situation can deteriorate quickly if breathing falters or complications like aspiration develop.
The Scale of the Problem
Alcohol-related poisoning deaths in the United States averaged nearly 21,800 per year during 2020 and 2021, a category that includes pure alcohol poisoning as well as cases where alcohol was combined with other substances. Total deaths from excessive alcohol use during that same period averaged over 178,000 annually, a 29% increase compared to just four years earlier. These numbers reflect both acute overdoses and the cumulative toll of long-term heavy drinking, including liver failure and its complications.