Alcohol and cancer risk: what you need to know

Experts weigh in on the risks of moderate drinking — and how people should assess them

Early this year, the US surgeon general issued a bombshell report. Before his term ended in January, Vivek Murthy warned that alcohol increases the risk of at least seven types of cancer and called for alcoholic drinks to carry cancer warning labels as cigarette packs do. The report¹ triggered a flurry of headlines about a risk that many people ignore.

The link between alcohol and cancer is not new. Scientists first labelled alcohol as a carcinogen more than 35 years ago, and the evidence has only grown since. “I don’t think the relationship between alcohol and certain cancers is terribly controversial,” says Tim Naimi, an alcohol epidemiologist at the University of Victoria, Canada.

What has been less clear is the level of drinking at which cancer risk starts to rise, and how to weigh that elevated risk against alcohol’s other impacts — in particular, the fiercely debated claim that low or moderate drinking improves heart health for some people. “Drinking a lot is absolutely bad,” says Emmanuela Gakidou, who analyses alcohol risks at the Institute for Health Metrics and Evaluation in Seattle, Washington. “It’s the low-to-moderate amount of alcohol where the debate is.”

All this information can leave people feeling confused. What exactly is low or moderate drinking? And how should people weigh up any enjoyment they might get from drinking with the risks? “A lot of people enjoy drinking, and they don’t like to be told that something they really enjoy is bad for you,” Gakidou says.

The cancer connection

For decades, scientists have been building a case that alcohol causes cancer. Because testing alcohol’s effects in randomized trials is difficult, they have instead drawn on other types of research. Several huge cohort studies that track many tens of thousands of people have found that alcohol is linked to higher rates of some types of cancer. One massive 2015 meta-analysis² — which pooled the results of hundreds of cohort studies and other research, including more than 485,000 cancer cases — found that risks of developing several cancers rose with the level of consumption.

But cohort studies have weaknesses. Researchers have to control for other confounding factors that could contribute to cancer risk, such as the fact that heavy drinkers are more likely to smoke than those who drink less, and people who drink small amounts of alcohol tend to have other healthy habits. And scientists know that study participants often say they drink less than they really do. Some studies attempt to adjust for this by inflating the levels of drinking to match those expected from alcohol sales, says Peter Sarich, an epidemiologist at the Daffodil Centre, a cancer research centre in Sydney, Australia.

Other evidence comes from animal experiments and from laboratory studies that have identified mechanisms by which alcohol can cause cancer. The most well-established is that ethanol is broken down into acetaldehyde, a toxin that damages DNA.

Scientists like to look at the sum of evidence, and there are two syntheses of research that are considered particularly definitive for alcohol and cancer. One is from the International Agency for Research on Cancer (IARC), the World Health Organization’s cancer branch. The agency classed drinking alcohol as carcinogenic to humans in 1988 — putting it in the same category as tobacco smoke and asbestos — and restated that view³ in 2007.

The other synthesis is from the Global Cancer Update Programme, a continuing effort to assess the totality of evidence on links between cancer, physical activity, body weight and diet (including alcohol) using systematic reviews and a panel of specialists to assess the strength of the evidence. The programme’s most recent report⁴, in 2018, concluded that there is strong evidence that alcoholic drinks increase the risk of several cancers.

“Because we have such a rigorous process, people have got confidence in the outcomes,” says Helen Croker, assistant director of research and policy at the World Cancer Research Fund International (WCRF) in London, which runs the project with several other cancer-research charities.

On the basis of such syntheses, researchers agree that alcohol increases the risk of at least seven types of cancer: mouth, throat (pharynx), voice box (larynx), oesophagus, breast cancer in women, colorectal and liver. Scientists suspect that other cancer types will be added as more data are collected.

Studies present this risk in different ways. One complication is that, in alcohol studies and guidelines, the amount of alcohol in a ‘standard’ drink differs between countries: a standard drink in the United States contains 14 grams of ethanol, in Canada it holds 13.45 g, in Australia 10 g and the in United Kingdom 8 g.

In the surgeon general’s report, the results from a large cohort study⁵ led by Sarich were used to calculate the absolute risks of developing any of the seven cancer types above (see ‘Cancer risk goes up’). This showed that around 17 out of 100 women who drink either nothing or less than one standard US drink per week (14 g ethanol) would develop such a cancer in their lifetime. This was taken as the baseline level of risk. This number rose to nearly 22 out of 100 in women who have two drinks per day (28 g ethanol) on average. Therefore, five more women would develop cancer owing to the higher level of alcohol consumption.

For men, the lifetime risk of developing one of these seven cancer types rose from a baseline of about 10 in 100 for those who don’t drink or have less than one standard drink per week, to about 13 out of 100 for those who, on average, have two drinks daily. The sex difference is because breast cancers account for a large number of cancers that are attributable to alcohol in women but not in men.

Rising risks

But what about people who drink very little? In the past few years, scientists have been zeroing in on the level of drinking at which cancer risk starts to rise. “We suspect that this question may have different answers for different cancer types,” says Jürgen Rehm, who studies the harms of alcohol and drugs at the Centre for Addiction and Mental Health in Toronto, Canada.

Rehm says the answer is clearest for breast cancer in women thanks in part to several large cohort studies that included many people who didn’t drink much. A systematic review⁶ of such studies, which was published last year, found that cancer risks were raised slightly in those who had half a drink (5 g of ethanol) per day — the lowest level examined. “And frankly, it probably doesn’t make a lot of sense to go down further, because I mean, who drinks one-quarter of a drink?” says Rehm.

In the 2018 Global Cancer Update Programme report⁴, the panel concluded that any amount of alcohol increased the risk of mouth, pharynx, larynx, oesophageal and breast cancer. For these conditions, “we find no lower threshold”, Croker says. The WCRF’s position, which is similar to that of the World Health Organization, is that there is no ‘safe’ level that does not increase overall cancer risk.

For some other cancer types, the panel found that elevated risks were most apparent at slightly greater levels of alcohol consumption. The risks of colorectal cancer increased above 30 g of ethanol per day (more than two US standard drinks), and liver cancer risks increased above 45 g per day (more than three US standard drinks). Lower levels of alcohol might still increase the risk of developing these cancers, but there wasn’t enough evidence to say.

The risks from low or moderate drinking can be hard to discern, partly because it is difficult for studies to define a reliable comparison group of people who don’t drink. Scientists know that some people stop drinking alcohol because their health is already declining: what researchers call sick-quitters. And some claim to be lifetime abstainers when records show they did drink in the past, says Naimi. Therefore, the ‘non-drinking’ group in a study might include unhealthy former drinkers.

“The whole evidence base around moderate drinking is scientifically frail — and that would be a charitable characterization,” Naimi says.

Alcohol versus smoking

A few years ago, Nick Sheron, a liver physician working at the University of Southampton, UK, did a quick calculation on the comparative risks of drinking and smoking. He said he was so surprised by the result that “I spent a long time trying to prove that I was wrong”. But he couldn’t — and epidemiologists he worked with agreed the numbers were right.

The study⁷ they published, in 2019, estimated that for non-smokers, drinking one bottle of wine per week was associated with an increased absolute lifetime cancer risk of 1% for men and 1.4% for women. (So, if 1,000 men and 1,000 women consumed a bottle of wine each week, an estimated 10 men and 14 women would develop cancer as a consequence.) This, they found, was equivalent to men smoking five cigarettes per week and women smoking ten. (Drinking three bottles per week was equivalent to men smoking eight cigarettes and women smoking 23 — more than a pack.)