After a heart attack and quadruple bypass surgery in 2010, Steve Colburn of Portland, Ore., began taking a cholesterol-lowering statin at the maximum dose. Soon, he began experiencing memory problems.

“Thinking and remembering became so laborious that I could not even recall my three-digit telephone extension or computer password at work,” said Mr. Colburn, 62, a sales representative and product developer. “All day, every day, I felt like my brain was mush.”

His doctor suggested a “drug vacation,” and when Mr. Colburn stopped taking the statin for six weeks, the problems disappeared. Then he tried a different statin at a high dose, but the cognitive difficulties returned. His doctor has since lowered his dose by more than half, and while the memory lapses have not disappeared, he has learned to cope.

“I felt like I didn’t have a choice to give up the drug,” Mr. Colburn said. “But I wanted to work with a dose that kept my numbers in an acceptable range and at the same time hopefully provided enough clarity of thinking that I could live with it.”

Statins are the most prescribed drugs in the world, and there is no doubt that for people at high risk of cardiovascular problems, the drugs lower not only cholesterol but also the risk of heart attack and stroke. But for years doctors have been fielding reports from patients that the drugs leave them feeling “fuzzy,” and unable to remember small and big things, like where they left the car, a favorite poem or a recently memorized presentation. Last week, the Food and Drug Administration finally acknowledged what many patients and doctors have believed for a long time: Statin drugs carry a risk of cognitive side effects. The agency also warned users about diabetes risk and muscle pain.

Nearly 21 million patients in the United States were prescribed statins last year, but nobody knows how many of them have experienced cognitive side effects. Dr. Beatrice Golomb, associate professor of medicine at the University of California, San Diego, has collected more than 3,000 reports of side effects related to statin use. She said doctors have too often dismissed the complaints, writing off the memory lapses and muscle pain, in particular, as a normal sign of getting older.

Many patients on statins also take medications for other health problems, which has made it difficult to discern whether statins are always to blame. For six years, Bill Moseley of Towson, Md., tried taking statins to lower his cholesterol; he also began taking medications for hypertension and high blood sugar. He found the drugs to be mind-numbing.

“I felt like a zombie in the afternoons,” he said.

While taking the cocktail of drugs, he began making driving errors. “I’d feel spacey and wouldn’t be in the right lane in enough time to make a turn,” he said. “Or I wouldn’t see someone I should be seeing. It was a feeling of detached, suppressed mental capability that should be there.”

Against his doctor’s advice, Mr. Moseley in 2006 stopped all of the drugs and began focusing on healthful eating and exercise, meeting regularly with a personal trainer and lifting weights. Four months later, the cognitive problems disappeared. Today, he is 69, his cholesterol has dropped from 225 to about 125, and his blood pressure and blood sugar are under control.

“I’m back to normal, and the more I work out over time, the better and better I get,” he said.

To be sure, millions of patients taking statins never experience cognitive side effects. John Hannon, 60, of Oceanside, N.Y., began taking a statin 20 years ago to lower his total cholesterol, which was about 270. Now his total cholesterol is in the 135 to 150 range.

“I’ve had no side effects that I’m aware of,” he said. “For me, it has been a wonder drug.”

Dr. Steven Nissen of the Cleveland Clinic noted that cognitive side effects have not been detected in randomized controlled trials of statin therapy. And even the warnings about muscle aches and diabetes need to be weighed against the fact that the drugs are proven to lower risk for heart attack and stroke, he said.

“For most physicians, and certainly for me, these warnings haven’t changed the decision-making process about who gets a statin and who doesn’t,” Dr. Nissen said.

Robert F. Hickey of Eagle, Colo., started taking statins in 2008 to lower his cholesterol, which was above 300. He also takes a myriad of other medications as a result of a kidney transplant. Last September, he began noticing memory problems and would sometimes go blank in the middle of memorized presentations.

“I began to notice, for a split second, difficulty with word recall,” said Mr. Hickey, a clinical psychologist and lecturer. “It was vocabulary that I use every day and had used for decades.”

His doctor put him through a battery of tests for early-onset dementia but found no signs of it. Instead, he suggested cutting the statin dose in half. Mr. Hickey said he hasn’t noticed any meaningful improvement yet, but the real test will come this week during a lecture in Las Vegas.

“I’ll have my hard copy with me, just in case,” he said.