Last week, the American Cancer Society published new guidelines that call for colorectal cancer screening to begin at age 45 — five years earlier than the group had previously recommended. But some experts are saying not so fast.

The new recommendation was made in reaction to increasing rates of colorectal cancer among people younger than 50.

But while the rise in cancers among this younger age group is troubling, this new recommendation was made before we know what’s behind the new trend. And it’s not clear that screening can help.

It makes intuitive sense to respond to increasing rates of colon cancer among young people by lowering the screening age, said Michael Hochman, director of the the Gehr Family Center for Health Systems Science at the Keck School of Medicine in Los Angeles. “But if I had a quarter for every time in medicine that we were tricked by an idea with intuitive appeal, I’d be a rich man,” Hochman said.

The ACS guidelines are based on models that use what’s known about the benefits of screening older populations to estimate how much screenings might help younger people. “We’re confident that the available data warrant this change in starting age,” said Richard Wender, chief cancer control officer at the American Cancer Society, who pointed out that the new guidelines rely on the most current data and models. But Hochman said the benefits to screening this younger group are still theoretical and we need studies to find out how useful the new recommendation is in practice. History has shown that more isn’t always better when it comes to cancer screening.

The idea behind cancer screening is that it prevents deaths and advanced cases of the disease by identifying cancers in early stages, while they’re more amenable to treatment and before they’ve spread to other areas of the body. If an abnormality like a polyp or adenoma (growths that are benign but may have the potential to become cancerous) is found during a colonoscopy, it can be removed on the spot and therefore prevented from progressing into something more nefarious. That’s the theory, anyway.

In practice, cancer screening hasn’t always proven quite so straightforward. Not every abnormality goes on to become cancer, so a screening can lead to treating things that could have been safely left alone. Wender told me that probably only about 1 in 10 polyps that doctors remove would have turned into a cancer, and the number is likely even lower for small polyps.

Which is why, despite its clear benefits, screening for colon cancer is not entirely benign, particularly when it’s done with a colonoscopy — the most common method of screening in the U.S. Colonoscopies are very often done with anesthesia in the U.S., making them essentially minor surgical procedures. A 2016 analysis found that for every 1,000 colonoscopies, about 16 people will end up hospitalized within a week. Large, randomized trials of the benefits of colonoscopies are still underway to determine the procedure’s effectiveness at preventing cancer deaths, but if the results show that colonoscopies are about as effective as sigmoidoscopies (a procedure that examines the rectum and lower colon through a small tube), “this means that you are about 8 times more likely to end up in the hospital within a week of your screening colonoscopy than you are to be saved from colon cancer death over 10 years,” Hochman and his colleagues wrote in a post to the Slow Medicine newsletter.

Meanwhile, the most aggressive cancers may spread before they can be detected in a less lethal form. Screening tends to disproportionately find the kinds of slow-progressing cancers that cause symptoms or become noticeable to doctors before they’re too advanced to treat (and in some cases the cancers may never become life-threatening), said Dartmouth physician and epidemiologist H. Gilbert Welch.

And while it may appear that colon cancer rates have fallen as screenings have become more common, the data suggests it’s more complicated than that.

In 2016, Welch and his colleagues published a paper looking at the relationship between colorectal cancer trends and screening and reported “unambiguous ‘good news’” — the overall incidence of colorectal cancer had dropped by more than 45 percent since its peak in the mid-1980s, and deaths have dropped too.

Screening became more widespread during this time period, and the United States Preventive Services Task Force calculates that for every 1,000 people age 50 to 75 who are screened according to current guidelines, between 17 and 24 cancer deaths will be averted. But Welch’s team concluded that screening alone can’t fully explain the drop in the number of cancer cases, because less than half the screening-age population was getting checked and much of the reduction happened before deaths would be expected to decline due to screening.

So why the drop in cancer rates? Welch’s group identified three additional possible explanations: better treatments, earlier recognition and detection of symptoms of colorectal cancer, and societal trends like reductions in smoking, less consumption of smoked meats and changes in the type and quantity of bacteria that live in people’s guts.

And that gets us back to the recent news that colorectal cancer is rising among the younger population. Just as something shifted to make overall rates drop, something also seems to have changed for people under 50 to make them more likely to develop colorectal cancer. “The risk is very much associated with the era in which a person was born,” Wender said. “People born in 1990 will now be at double the risk of colorectal cancer and over four times the risk of rectal cancer before the age of 50 compared to people born in 1950.”

Those sound like scary numbers, but in absolute terms, the risks are still quite small, and that’s important, because the risk-to-benefit ratio will shift as you screen a population that has lower rates of the disease, said Rita Redberg, a physician at University of California San Francisco and editor of JAMA Internal Medicine.

The rising risk of colorectal cancer among young people today happened faster than you’d expect if it were caused by genetics, so the cause is more likely to be environmental, Wender said. Although about a third of colorectal cancers can be traced to family history, two-thirds cannot be explained by known risk factors.

Whether these new cancers will be as amenable to screening as those of older generations remains unproven. “Colon cancer is often more aggressive in younger populations,” Hochman said. “The aggressive ones are the hardest to detect when treatable. They may be metastatic” — spreading to other parts of the body — “virtually the same time that they form.” In that case, screening probably won’t help.

So screening has uncertain benefits for this younger population, and it comes with some known risks. Anesthesia and other risks related to surgery can be reduced if colonoscopies are reserved for following up on positive results from less-invasive tests rather than being used as a first-line screening test, as the Canadian Task Force on Preventive Health Care advises. The ACS guidelines present an array of six tests for screening, but they do not make a recommendation about which test is best because the ACS views each as equally acceptable and believes that encouraging patients to choose from among all the options will improve screening rates. “The best test is the one that gets done,” Wender said.

There are lots of ways to get your colon checked The six colorectal cancer screening tests recommended by the American Cancer Society and how often they should be performed test frequency Fecal immunochemical test (FIT) annually Guaiac‐based fecal occult blood test (gFOBT) annually Multitarget stool DNA test every 3 years CT colonography every 5 years Flexible sigmoidoscopy every 5 years Colonoscopy every 10 years Source: American Cancer Society

So what’s a 45-year-old to do? The answer to that will depend on that person’s individual circumstances and their value judgements about the risks and benefits. These screenings vary in price — a colonoscopy can cost $1,000 or more while a fecal immunochemical test (which looks for hidden blood in a stool sample) costs only about $30 — and they involve varying levels and types of discomfort. But holding off on screening for a few years is a defensible choice, too. Although colorectal screening has been shown to reduce deaths from these cancers, it has never been shown to reduce overall mortality, and screenings can lead to a spiral of further unnecessary and costly tests. Hochman is a primary care physician and said that if a patient wants to decline colon cancer screening, he doesn’t try to talk them out of it. “People have a reasonable right to decline it,” he said, adding that saying no isn’t necessarily an irrational choice. Sensible people may say “it’s not worth it to them.”

Read more: “You Could Skip Your Colonoscopy If You’re Willing To Collect Your Poop”