If you're not on a statin, you probably know someone who is. These cholesterol-lowering drugs are among the most widely prescribed medications in the United States, with millions of people taking them to prevent heart attacks and strokes. According to the Food and Drug Administration (FDA), the value of statins in cardiovascular disease prevention is clearly established. But the drugs have some surprising downsides, including an increased risk for diabetes.

Doctors once joked about putting statins in the water supply because so many people appear to need them. Under guidelines released in 2013, some 56 million Americans are now eligible for the drugs based on their cardiovascular risk factors. That's up from 43 million before the new guidelines.

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Four groups should consider statin therapy, according to the American Heart Association and the American College of Cardiology, who issued the guidelines. They are:

  • People who have cardiovascular disease
  • People with an LDL or ''bad'' cholesterol of 190 mg/dL or higher
  • People with Type 2 diabetes and age 40 to 75
  • People age 40 to 75 who have an estimated 10-year risk of cardiovascular disease of 7.5 percent or higher

If your doctor has suggested you start taking a statin, here's information to consider when making the decision.

Why your doctor may prescribe a statin

People who’ve had a heart attack are likely to be put on a statin. For others, looking at individual risk factors is especially important before recommending one, says Ravi Dave, MD, professor of medicine at University of California, Los Angeles and director of interventional cardiology at UCLA Santa Monica Medical Center.

"A lot of doctors are using the 10-year risk calculator recommended by the American Heart Association," Dave says. If you know your cholesterol numbers, you can calculate your own risk online.

If your risk is more than 7.5 percent, your doctor may recommend a statin and discuss the pros and cons.

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Statins: The benefits

A lower risk of heart attack and stroke. The drugs work by slowing down your body's production of cholesterol. Lowering cholesterol reduces the risk of heart disease and stroke. "Statins can lower premature death by 20 percent, cardiovascular disease by about 50 percent and stroke by 50 percent,'' Dave says, citing clinical research.

Better erections. Men on statins had better erectile function than those not taking the drugs, according to a recent analysis that looked at 11 published studies. As cholesterol levels declined, the benefit on erectile functioning improved, regardless of the man's age, the researchers reported. That makes sense, Dave says. Statins improve blood flow, and that makes the erections better.

Statins: The risks

Serious liver problems. "The most important [of potential risks] are liver side effects,'' Dave says. The risk is small, according to Dave and the FDA. ''The risk of having serious liver problem is about 1 percent," he says. While that percentage may sound tiny, keep in mind that 1 percent of 56 million people is 56,000 people. Before starting a statin you should have a liver function test, Dave says. Once on the drug, you should have the test again six weeks to three months later, he says.

In the past, doctors were advised to test patients' liver functioning regularly, but recently the FDA decided less testing is acceptable. Liver tests should be done before starting a statin and then ''as needed if there are symptoms of liver damage," the FDA says. Symptoms include loss of appetite, right upper abdominal discomfort, dark urine or yellowing of skin and eyes. If you do develop a liver problem, Dave says, stopping the medication reverses the problem.

Muscle pain. Muscle side effects affect about 20 percent of patients, Dave says, mostly in the form of aches and pains. Serious muscle injuries are rarer, affecting about 1 percent of statin users, Dave says. Again, the problems typically reverse once the drugs are stopped.

An increased risk for diabetes. Taking statins is linked with a higher risk of developing diabetes, according to a study in BMJ in 2013. Researchers looked at 1.5 million men and women who started statin therapy between 1997 and 2010. The risk varied from drug to drug. People taking atorvastatin (Lipitor) had a 22 percent increased risk compared to those on pravastatin (Pravachol). Pravastatin was used as the comparison since it has been shown to have good effects on diabetes in those newly diagnosed. People on fluvastatin (Lescol) had a 5 percent decreased risk. The researchers suggest doctors should consider preferential use of pravastatin and possibly, fluvastatin.

Memory issues. Studies have yielded conflicting results, with some researchers finding an apparent link between statin use and memory problems and other cognitive issues and other studies finding none. According to the FDA, “the agency has reviewed databases that record reports of bad reactions to drugs and statin clinical trials that included assessments of cognitive function. The reports about memory loss, forgetfulness and confusion span all statin products and all age groups.” The FDA calls these experiences “rare” but says, “those affected often report feeling ‘fuzzy’ or unfocused in their thinking.” It reports that the symptoms were generally reversed within a few weeks after stopping the drug.

A false sense of security. Statins may give users a false sense of security, UCLA researchers found. Statin users today, compared to a decade ago, ate about 180 more calories and 9 more grams of fat a day, they reported in JAMA Internal Medicine. Excess weight is a risk factor for heart disease. Just because you’re taking a statin doesn’t mean a heart-healthy diet and exercise aren’t still important.

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Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.