You might assume that standard medical advice was supported by mounds of scientific research. But researchers recently discovered that nearly 400 routine practices were flatly contradicted by studies published in leading journals.

Of more than 3,000 studies published in JAMA, the Lancet and the New England Journal of Medicine, more than 1 in 10 amounted to a “medical reversal”: a conclusion opposite of what had been conventional wisdom among doctors. Dr. Vinay Prasad of Oregon Health and Science University, said, “Very smart and well-intentioned people came to practice these things for many, many years. But they were wrong.”

Here are 10 findings that contradict once widely held theories.

• Peanut allergies occur whether or not a child is exposed to peanuts before age 3. Pediatricians have counseled parents to keep babies away from peanuts for the first three years of life. As it turns out, children exposed to peanuts before they were even 1-year-old have no greater risk of peanut allergies.

• Fish oil does not reduce the risk of heart disease. Fatty fish contains omega-3 fatty acids. Omega-3 supplements lower levels of triglycerides, and high levels of triglycerides are linked to an increased risk of heart disease. Not to mention that omega-3 fatty acids seem to reduce inflammation, a key feature of heart attacks. But in a trial involving 12,500 people at risk for heart trouble, daily omega-3 supplements did not protect against heart disease.

• A lifelike doll carried around by teenage girls will not deter pregnancies. A study found that girls who carried “infant simulators” were slightly more likely to become pregnant than girls who did not get the dolls.

• Ginkgo biloba does not protect against memory loss and dementia. The supplement is still is promoted as a way to preserve memory. But a large federal study, published in 2008, definitively showed the supplement is useless for this purpose.

• To treat emergency room patients in acute pain, a single dose of oral opioids is no better than drugs like aspirin and ibuprofen, a clinical trial discovered.

• Testosterone treatment does not help older men retain their memory. Some men have low levels of testosterone and memory problems, and older men with higher testosterone levels seemed to do better on tests of mental functioning. But a rigorous clinical trial showed that testosterone was no better than a sugar pill in helping older men avoid memory loss.

• To protect against asthma attacks, it won’t help to keep your house free of dust mites, mice and cockroaches. The theory was that allergic reactions to them can trigger asthma attacks. But intensive pest management in homes with children sensitized to mouse allergens did nothing to reduce the frequency of their asthma attacks, researchers reported in 2017.

• Step counters and calorie trackers do not help you lose weight. Among 470 dieters followed for two years, those who wore such tracking devices actually lost less weight than those who just followed standard advice.

• Torn knee meniscus? Try physical therapy first, surgery later. An estimated 460,000 U.S. patients get surgery each year to fix knee cartilage that tears. The tear is painful, and many patients fear that if it is not surgically treated, the pain will linger. But when patients with a torn meniscus and moderate arthritis were randomized to six months of physical therapy or surgery, both groups improved, and to the same extent.

• If a pregnant woman’s water breaks prematurely, the baby does not have to be delivered immediately. Obstetricians worried that bacteria could invade what had been a sterile environment around the fetus, causing infection. But a clinical trial found that if obstetricians carefully monitor the fetus while waiting for labor to begin naturally, the fetus is at no greater risk for infection. And newborns left to gestate were healthier, with less respiratory distress and a lower risk of death, than those who were delivered immediately after a break.