The U.S. Surgeon General proposed on Friday adding a label to bottles of alcoholic beverages warning of the cancer risks associated with drinking them, but how big is the risk and which cancers are most linked to drinking?

According to the advisory, less than half of Americans know of the cancer risks posed by drinking alcohol.

Alcohol is a leading cause of cancer, U.S. Surgeon General Vivek Murthy said in his advisory, which came as research and evidence mounts about the bad effects that alcohol has on human health.

Moderate drinking was once thought to have benefits for the heart, but better research methods have thrown cold water on that.

Here's what you should know:

What's the harm in drinking alcohol?

Drinking raises the risk of several types of cancer, including colon, liver, breast and mouth and throat.

The seven cancers most linked to drinking include: mouth, throat, voice box, esophagus, liver, breast, and colon and rectum, according to the National Cancer Institute.

According to Murthy’s advisory, there are about 100,000 alcohol-related cancer cases and about 20,000 alcohol-related cancer deaths annually in the United States.

“As you consider whether or how much to drink, keep in mind that less is better when it comes to cancer risk,” Murthy wrote Friday on the social media platform X.

"Further, the data in humans on alcohol and health show a strong association between drinking alcohol and increased cancer risk, regardless of the type of alcohol," the advisory states.

Why does alcohol pose a cancer risk?

According to the advisory, studies have found four possible ways drinking alcohol can lead to cancer, though research is still being conducted on some.

The main two ways include:

• Alcohol breaks down in the body into a substance called acetaldehyde, which can damage your cells and stop them from repairing themselves. That creates the conditions for cancer to grow.
• Alcohol also "generates reactive oxygen species," which can increase inflammation and damage DNA, proteins and lipids.

Others ways that are still being studied:

• Alcohol can alter hormone levels like estrogen, which can impact the development of breast cancer.
• Carcinogens like those from tobacco smoke can dissolve in alcohol, allowing them to be better absorbed in the body. This can increase the risk for mouth and throat cancers.

What about moderate drinking?

That idea that moderate drinking has health benefits came from imperfect studies comparing groups of people by how much they drink. Usually, that was measured at one point in time. And none of the studies randomly assigned people to drink or not drink, so they couldn’t prove cause and effect.

People who report drinking moderately tend to have higher levels of education, higher incomes and better access to health care, said Dr. Timothy Naimi, who directs the Canadian Institute for Substance Use Research at the University of Victoria in British Columbia.

“It turns out that when you adjust for those things, the benefits tend to disappear,” he said.

Another problem: Most studies didn’t include younger people. Almost half of the people who die from alcohol-related causes die before the age of 50.

“If you’re studying people who survived into middle age, didn’t quit drinking because of a problem and didn’t become a heavy drinker, that’s a very select group,” Naimi said. “It creates an appearance of a benefit for moderate drinkers that is actually a statistical illusion.”

Other studies challenge the idea that alcohol has benefits. These studies compare people with a gene variant that makes it unpleasant to drink to people without the gene variant. People with the variant tend to drink very little or not at all. One of these studies found people with the gene variant have a lower risk of heart disease — another blow to the idea that alcohol protects people from heart problems.

What do guidelines say?

Thousands of U.S. deaths per year could be prevented if people followed the government’s dietary guidelines, Naimi said.

Those guidelines advise men to limit themselves to two drinks or fewer per day and women to one drink or fewer per day. One drink is the equivalent of about one 12-ounce can of beer, a 5-ounce glass of wine or a shot of liquor.

But guidelines vary a lot from country to country. Still, the overall trend is toward drinking less.

The United Kingdom, France, Denmark, Holland and Australia recently reviewed new evidence and lowered their alcohol consumption recommendations. Ireland will require cancer warning labels on alcohol starting in 2026.

“The scientific consensus has shifted due to the overwhelming evidence linking alcohol to over 200 health conditions, including cancers, cardiovascular diseases and injuries,” said Carina Ferreira-Borges, regional adviser for alcohol at the World Health Organization regional office for Europe.

Naimi served on an advisory committee that wanted to lower the U.S. recommendation for men to one drink per day. That advice was considered and rejected when the federal recommendations came out in 2020.

“The simple message that’s best supported by the evidence is that, if you drink, less is better when it comes to health," Naimi said.

Could warning labels be added?

Bottles of beer, wine and liquor already carry warning labels that say pregnant women should not drink and that alcohol consumption can impair someone's ability to drive a car. But Murthy's proposed label would go even further, raising awareness about the risk for cancer, too.

Even with the Surgeon General's advisory and new research that shows the dangers of drinking, it's unlikely Congress would act swiftly to enact a new Surgeon General's warning on alcohol products.

It's been nearly four decades since Congress approved the first government warning label on alcohol, the one that says pregnant women shouldn't drink and warns about the dangers of driving while drinking. No updates have been made since then.

Before that, Congress approved a label on cigarettes cautioning users that smoking is hazardous to health, a move that is credited with helping America substantially reduce its bad habit.

Any effort to add a cancer warning label to alcohol would face significant push back from a well-funded and powerful beverage industry, which spends nearly $30 million every year lobbying Congress.

Just weeks ago, the federal government passed on altering the existing dietary guidelines around alcohol. The current guidelines recommend women have one drink or fewer per day while men stick two or fewer.