The biphasic effect of alcohol refers to the initial pleasant feelings you experience after drinking followed by its depressant effect.
When you drink alcohol, you may initially feel happy and stimulated. When you keep drinking, these effects do not multiply, however. Instead, you will often feel the depressant and sedative effects. This is the biphasic effect.
It feels counterintuitive as it seems that the more you drink, the better you should feel. Once you pass the point of “diminished returns,” often (but not always) around a BAC (blood alcohol content) above 0.05%, you will start to feel the depressant effects. After all, alcohol is a depressant substance.
Distinct Phases of the Biphasic Effects of Alcohol
The biphasic effects of alcohol consumption occur in two separate phases and represent the curve between the “good” effects of alcohol and the “bad.”
About Phase I
Alcohol can start to affect the brain and body within minutes of consuming it. During the first phase of the biphasic effect, it can feel good. You may feel alert, talkative, warm, stimulated, and happy when you start drinking alcohol. This typically occurs at BAC below 0.05%.
After continuing to drink, usually around a BAC of .05% or greater (although this varies widely between individuals depending on your genetics and your tolerance) you will enter phase II.
About Phase II
During phase II, alcohol’s depressant and sedative effects kick in. You can start to feel tired and drowsy and may become impaired. Things like balance and coordination can feel off, and it can be harder to think clearly, remember things, or focus.
In the United States, the legal definition of intoxication is a BAC of 0.08%. It is a crime to drive with a BAC at this level or above.
How is alcohol metabolized in the body?
When alcohol is consumed, it is absorbed through the small intestine and stomach. Then, it is passed into the bloodstream where it is quickly distributed to your brain, lungs, kidneys, and liver. Your liver then works to break down the alcohol into non-toxic components, which are then past back into the bloodstream and eventually excreted by the kidneys.
It typically takes an hour for your liver to break down the alcohol in one standard drink (one beer, one glass of wine, or one shot of alcohol). How quickly your body is able to metabolize alcohol depends on several factors, including your age, weight, sex, race, and other medical conditions. It also depends on how much and how fast you drank, how much food is in your system, and any medications you’re taking.
What symptoms can I expect at various BACs?
Based on BAC levels, the following effects can occur when drinking alcohol (note these are estimates and vary widely between individuals):
- BAC of up to 0.05%: mild impairment of coordination and balance as well as relaxation
- BAC between 0.06% and 0.15%: more impairment of motor skills, decision-making abilities, speech, memory, cognitive function, and attention as well as drowsiness and sedation
- BAC between 0.16% and 0.30%: severe impairment of reaction time, balance, coordination, judgment, speech, memory, and cognition along with potential blackouts (memory loss), loss of consciousness, or vomiting
- BAC between 0.31% and 0.45%: risk of potential life-threatening alcohol overdose as life-sustaining functions are dangerously suppressed
Risks of Acute Alcohol Use
The more alcohol you drink at a time, the higher the potential risks. Drinking too much too fast can lead to an alcohol overdose, which can cause coma or even death from alcohol poisoning. Additional risks of alcohol intoxication include the following:
- Injury or accident
- Higher odds of being involved in or a victim of a crime
- Risky sexual behaviors that can lead to the development of an infectious disease or unwanted pregnancy
- Higher rate of alcohol misuse with repetition 
People who are sensitive to the biphasic effects of alcohol are also at a higher risk for developing alcohol use disorder.
Risks of Chronic Alcohol Use
Long-term effects of heavy drinking can include the following:
- Digestive issues
- Increased risk for stroke and heart disease
- Liver disease
- Lowered immune system
- Anxiety and depression
- Risk for alcohol-related dementia and memory issues
- Heightened risk for developing many forms of cancer
- Social and relationship issues as well as problems at work, school, or home
- Alcohol tolerance, dependence, and alcohol use disorder
What if You Are a Moderate Drinker, Heavy Drinker, or Alcoholic?
Drinking regularly increases tolerance for alcohol, which can shift the biphasic effect some. It can take more alcohol to reach the top of the bell curve and progress to phase II. Moderate or heavy drinkers often remain in phase I longer and suffer fewer of the negative consequences of phase II.
While this may seem like a pay off in the short term, it can lead to serious consequences long term: Regular heavy drinking can lead to serious health consequences, development of irreversible chronic health conditions such as liver disease, dementia, and heart failure, and can lead to alcohol use disorder (AUD) and rarely a syndrome of severe or even life-threatening withdrawal.
The CDC defines the following drinking levels:
- Moderate: 2 drinks or less in a day for a man or 1 drink or less for a woman
- Heavy drinking: 15 drinks in a week for a man or 8 drinks in a week for a woman
- Alcohol use disorder: regular patterns of excessive drinking as well as compulsive drinking, alcohol cravings, alcohol dependence, and mental, physical, and social issues related to drinking
The bottom line: Understanding a little more about the biphasic effects of alcohol can help us all to look at our own drinking patterns, how they change over time, and whether or not we might be developing more concerning symptoms. If you have concerns about your drinking or that of a loved one, reach out to your medical team for support.