A study published Monday adds to the evidence, finding a modest risk among older women who used a variety of statins.
It's a puzzling link, and specialists say people who most need statins because of a high risk for a heart attack should stick with the drugs.
"What I fear here is that people who need and will benefit from statins will be scared off of using the drugs because of reports like this," says Dr. Steven Nissen, cardiology chairman at the Cleveland Clinic, who was not involved with the research. "We don't want these drugs in the water supply, but we want the right people treated. When they are, this effect is not a significant limitation."
But more and more doctors are urging otherwise healthy people to use the pills as a way to prevent heart disease. For them, the findings add another potential complication as they consider whether to tackle their cholesterol with diet and exercise alone or add a medication.
"The statin should not be seen as the magic pill," says Dr. Yunsheng Ma of the University of Massachusetts Medical School, who led the study of postmenopausal women.
Statins are one of the most widely prescribed drugs, and among the most touted with good reason. They can dramatically lower so-called "bad" LDL cholesterol. Studies make clear that they save lives when used by people who already have heart disease.
What is debated is how much the drugs help people who do not yet have cardiovascular disease but whose chances are higher because of other factors such as smoking or high blood pressure — or diabetes. In fact, long-term diabetes is so heart-risky that the American Diabetes Association urges fairly aggressive statin use by many diabetics. For everyone else, Nissen says the general rule is statins help people who have at least a 10 per cent chance of a heart attack in the next 10 years, something a doctor can calculate.
All drugs have side effects that are important to consider while deciding whether they are a good bet for an individual. Statins have long been known to cause muscle pain that on rare occasions becomes a serious breakdown of muscle that can lead to kidney failure, even death.
But whether statins can make blood sugar rise enough that someone crosses the threshold to diabetes has been confusing.
After all, some of the same risks for heart disease — such as being overweight and sedentary — also increase the odds of developing Type 2 diabetes. And Ma says too many statin users wrongly assume the pills will let them eat whatever they want.
Ma's team examined a huge U.S. government study that tracked the health of postmenopausal women for many years. They culled the records of more than 153,000 women who did not have diabetes when they enrolled in the Women's Health Initiative in the 1990s. Just seven per cent were taking statins at the time.
Fast forward to 2005: Nearly 10 per cent of the statin users had developed diabetes, compared with 6.4 per cent of the older women who hadn't used the drugs at the study's start, Ma concluded. The findings were reported online Monday by the journal Archives of Internal Medicine.
This is what scientists call an observational study, which can hint at a risk but can't prove it.
But it comes after a number of smaller but more precise studies — where patients were randomly assigned to take a statin or some other treatment — that also have found a link. The first to prompt doctor head-scratching was in 2008, a study of the drug Crestor. Last June, a report in the Journal of the American Medical Association analyzed five additional randomized trials and concluded the increased risk was small but real for people taking higher doses of any statin.
That report calculated that one fewer patient would experience a heart attack or other cardiovascular problem for every 155 patients treated for a year — and there would be one additional case of diabetes for every 498 patients treated.
At the National Institutes of Health, diabetes specialist Dr. Judith Fradkin says statins' benefits outweigh the potential side effect, and that newly developed diabetes won't harm right away.
"The danger here is alarming people and having them go off a medication that's of proven benefit," she says.
But Dr. Beatrice Golomb of the University of California, San Diego, welcomed the new study as a needed note of caution for women, saying there's less certainty about the drugs' overall effects in them. Stay tuned: Her own research aims to narrow down which statin users are more likely to experience a blood-sugar jump.