Portside

You probably know about the surge in childhood peanut allergies. Peanut allergies in American children more than tripled between 1997 and 2008, after doctors told pregnant and lactating women to avoid eating peanuts and parents to avoid feeding them to children under 3. This was based on guidance issued by the American Academy of Pediatrics in 2000.

You probably also know that this guidance, following similar guidance in Britain, turned out to be entirely wrong and, in fact, avoiding peanuts caused many of those allergies in the first place.

That should not have been surprising, because the advice violated a basic principle of immunology: Early exposure to foreign molecules builds resistance. In Israel, where babies are regularly fed peanuts, peanut allergies are rare. Moreover, at least one of the studies on which the British advice was based showed the opposite of what the guidance specified.

As early as 1998, Gideon Lack, a British pediatric allergist and immunologist, challenged the guidelines, saying they were “not evidence-based.” But for years, many doctors dismissed Dr. Lack’s findings, even calling his studies that introduced peanut butter early to babies unethical.

When I first reported on peanut allergies in 2006, doctors expressed a wide range of theories, at the same time that the “hygiene hypothesis,” which holds that overly sterile environments can trigger allergic responses, was gaining traction. Still, the guidance I got from my pediatrician when my second child was born that same year was firmly “no peanuts.”

It wasn’t until 2008, when Lack and his colleagues published a study showing that babies who ate peanuts were less likely to have allergies, that the A.A.P. issued a report, acknowledging there was a “lack of evidence” for its advice regarding pregnant women. But it stopped short of telling parents to feed babies peanuts as a means of prevention. Finally, in 2017, following yet another definitive study by Lack, the A.A.P. fully reversed its early position, now telling parents to feed their children peanuts early.

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But by then, thousands of parents who conscientiously did what medical authorities told them to do had effectively given their children peanut allergies.

This avoidable tragedy is one of several episodes of medical authorities sticking to erroneous positions despite countervailing evidence that Marty Makary, a surgeon and professor at Johns Hopkins School of Medicine, examines in his new book, “Blind Spots: When Medicine Gets It Wrong, and What It Means for Our Health.”

Rather than remaining open to dissent, Makary writes, the medical profession frequently closes ranks, leaning toward established practice, consensus and groupthink. (Makary acknowledges having been guilty of this himself.)

In this kind of environment, what begins as opinion can quickly become dogma, especially once the profession has become wed to an idea.

“There was an internal policy that all A.A.P. recommendations had to be consistent,” a member of the committee that issued the original peanut guidance told Makary. “It was old dogma, perpetuated.” In other words, Makary writes, “protecting the institution was more important than letting the public see alternative viewpoints.”

The causes of this perpetuated mistake replicate those of other medical errors outlined in Makary’s book.

In 1983, near the beginning of the AIDS crisis, the American Red Cross, the American Association of Blood Banks and the Council of Community Blood Centers rejected a recommendation by a high-ranking C.D.C. expert to restrict donations from people at high risk for AIDS. Instead, they issued a joint statement insisting that “there is no absolute evidence that AIDS is transmitted by blood or blood products.” The overriding concern was that Americans would not trust the blood supply, or donate blood, if people questioned its safety.

As with the advice on peanuts, a reversal came about far later than it should have. It took years for the blood banking industry to begin screening donors and it wasn’t until 1988 that the F.D.A. required all blood banks to test for H.I.V. antibodies. In the interim, half of American hemophiliacs, and many others, were infected with H.I.V. by blood transfusions, leading to more than 4,000 deaths.

In the case of hormone replacement therapy, millions of women used H.R.T. to relieve the symptoms of menopause and decrease the likelihood of bone fractures, heart attacks and even Alzheimer’s disease in later years. But in 2002, women were told unequivocally that taking estrogen and progestin to treat menopause symptoms increased their risk of breast cancer. The guidance that formed the basis of that recommendation was based on a single study, even though the lead author of that study acknowledged to Makary years later that it did not show a statistically significant difference in rates of cancer among women who were on H.R.T. and those on a placebo.

As with peanuts, early dissenters were ignored, ridiculed and suffered professional consequences. It would be years before the guidance was corrected. In the interim, millions of women suffered withdrawal from hormone therapy and missed out on H.R.T.’s health benefits.

While these mistakes are appalling, more worrisome are the enduring root causes of those errors. Medical journals and conferences regularly reject presentations and articles that overturn conventional wisdom, even when that wisdom is based on flimsy underlying data. For political or practical reasons consensus is often prized over dissenting opinions.

“We’re seeing science used as political propaganda,” Makary told me when I spoke to him by phone. But, he argues, mistakes can’t be freely corrected or updated unless researchers are encouraged to pursue alternative research.

“Asking questions has become forbidden in some circles,” Makary writes. “But asking questions is not the problem, it’s the solution.”

With trust in science on the wane, conspiracy theories and misinformation proliferating and anti-vaxxers like Robert Kennedy Jr. setting a deranged example, this may not seem like the best time to criticize the medical profession. Yet a dose of healthy skepticism may be the healthiest attitude when information seems contradictory, whether it’s about a decades-long practice or newer, faddish procedures like tongue-tie surgery.

When it comes to medical certainty in the face of dissent, it’s useful to remember the case of Ignaz Semmelweis. Semmelweis was the Hungarian doctor who in 1847 proposed that doctors wash their hands before delivering babies, to reduce the chances of “childbed fever,” now known as postpartum infection. For making the offensive suggestion that doctors’ hands might somehow be unclean, Semmelweis was denounced by the medical establishment and later lost his job. His life ended at age 47 in an insane asylum and on an especially cruel note. He died there of sepsis, the very disease he’d fought so hard against as a doctor.

Pamela Paul is an Opinion columnist at The Times, writing about culture, politics, ideas and the way we live now.