If you’re celebrating the holidays with family and friends, there’s a good chance alcohol will be part of the picture. You might like to ring in the New Year with a champagne toast. You may think a drink or two helps take the edge off stressful family gatherings. Or maybe football, friends and beer are one of your favorite parts of winter. People drink for many reasons. But as everyone knows, if you overdo it there’ll be little to celebrate the next day.

Why does alcohol feel so good in small amounts but so lousy if you drink too much? Why do some people develop drinking problems while others don’t? Scientists have been working hard to learn why people use, abuse and sometimes become addicted to alcohol. Although there’s still much to learn, this research has already led to better ways to treat and prevent alcohol-related problems.

Alcohol use is common in this country. About two-thirds of American adults had at least 1 drink during the past year, according to an NIH survey.

Occasional, moderate drinking usually poses few problems. However, more than 1 in 10 adults grapple with alcohol dependence, or alcoholism, at some time in their lives. Nearly 1 in 5 struggle with alcohol abuse—harmful drinking that leads to missing work, neglecting family responsibilities or drinking in dangerous situations, like when driving. Long-term heavy drinking can damage the liver and cause several types of cancer, inflammation of the pancreas and brain damage.

When it comes to holiday drinking, the consequences can range from making an embarrassing remark to being arrested for drunken driving or causing a deadly traffic accident.

“The main problem with holiday drinking is that people are often drinking for longer periods of time than they normally do, and they’re staying up later than they normally do. They may not have a good frame of reference for how the alcohol will affect them,” says Dr. Dennis Twombly, a scientist at NIH’s National Institute on Alcohol Abuse and Alcoholism.

Low levels of alcohol can act as a stimulant. “At low doses, alcohol has effects that the brain perceives as positive and rewarding,” Twombly explains. “It can cause euphoria and relieve anxiety and stress.”  Scientists today are working to discover what parts of the brain are affected by alcohol and how it creates these pleasurable feelings.

At higher levels, alcohol’s impact on the brain begins to take a downturn. It can act as a depressant and make you sleepy. Twombly says, “You start to see effects on other areas of the brain like the cerebellum, which causes people to lose their balance and coordination. Their reaction times may become slower. Their ability to speak may become somewhat impaired.”  Inhibition and judgment are also affected, and emotions can become unstable.

Alcohol quickly moves from your stomach into your bloodstream, where it travels to all your major organs and tissues. Eating before you drink helps slow down this process. When alcohol reaches your liver, it gets broken down and converted to other substances. Liver enzymes, however, can only break down about half of an alcohol-containing beverage per hour.

If you drink alcohol faster than your liver can clear it, the alcohol levels in your blood will climb. Binge drinking in particular—for men, defined as 5 or more drinks over a 2-hour period; for women, 4 or more—can quickly raise your blood alcohol above the legal limit. This excess alcohol continues to circulate throughout the body long after your last drink, affecting your heart, brain and other organs.

How much alcohol is too much? That depends. Alcohol affects everyone differently. Researchers do know that drinking beyond a certain amount increases your risk for alcohol abuse or dependence. The risk increases substantially for men who have more than 4 drinks in a single day or more than 14 per week, and for women who have more than 3 drinks in a day or more than 7 per week. A drink is generally defined as a 12-ounce bottle of beer or wine cooler, a 5-ounce glass of wine, or a 1.5 ounce shot of 80-proof liquor.

Scientists are working to understand why some people develop long-term problems with alcohol. Researchers have long known that alcoholism tends to run in families.In fact, people with an alcoholic parent are about 4 times more likely than others of developing an alcohol use disorder. But, despite their increased risk, many children of alcoholics do not become alcoholic themselves.

“We know from research that roughly half the risk for alcoholism can be explained by some genes,” Twombly says. Nearly a dozen alcoholism-related genes have already been identified, and many more are expected. “The other half of the risk,” he says, “has to do with the environment, including your family and friends, your anxiety and stress levels and even your childhood experiences.”

For holiday revelers, or anyone who drinks to excess in a single evening, the next day is likely to bring great discomfort in the form of a hangover. Painkillers like aspirin may help with headaches, but don’t take acetaminophen, a common alternative. The drug can interact with alcohol and damage the liver.

Drinking coffee won’t help, either. Twombly explains, “It might help with drowsiness, but it will have no effect whatsoever on how intoxicated you are or how rapidly the alcohol is absorbed or eliminated from the body.”

The only real cure for a hangover is time, Twombly says. “Sleeping it off, eating a little and drinking non-alcoholic beverages can help. But you basically have to wait for the alcohol and its by-products to be cleared from your system.”

If you get a hangover over the holidays, let it inspire a New Year’s resolution—to drink responsibly and moderately in the coming year.