When she was a young physician, Dr. Martha Gulati noticed that many of her mentors were prescribing vitamin E and folic acid to patients. Preliminary studies in the early 1990s had linked both supplements to a lower risk of heart disease.

She urged her father to pop the pills as well: “Dad, you should be on these vitamins, because every cardiologist is taking them or putting their patients on [them],” recalled Dr. Gulati, now chief of cardiology for the University of Arizona College of Medicine-Phoenix.

But just a few years later, she found herself reversing course, after rigorous clinical trials found neither vitamin E nor folic acid supplements did anything to protect the heart. Even worse, studies linked high-dose vitamin E to a higher risk of heart failure, prostate cancer and death from any cause.

Dr. Gulati told her father he “might want to stop taking” the vitamins.

More than half of Americans take vitamin supplements, including 68 percent of those age 65 and older, according to a 2013 Gallup poll. Among older adults, 29 percent take four or more supplements of any kind, according to a Journal of Nutrition study published in 2017.