Not long ago, I cared for a middle-aged attorney who had a sarcoma. This kind of cancer arises from connective tissues like muscle and bone; if confined, it usually can be cured. But in this woman’s case, the malignancy was discovered after it had spread from her thigh to her lungs and liver. She initially was treated with chemotherapy, and the tumors shrank; but a year later, they grew again.

My patient told me that she was a clear-eyed and rational person who made decisions based on facts: she would do so about her treatment, much the way she did in her practice as a lawyer. She underwent a series of increasingly arduous therapies, but her condition worsened as the cancer grew in vital organs, and she felt increasingly desperate. Friends, with what she acknowledged as good intentions, sent her reports from the Internet of “cancer cures,” which included cleansing her body of toxins with coffee enemas, ingesting solutions made from Chinese herbs, passing her plasma over resins. At one of our last appointments, my patient recounted how hard it was not to give in and chase illusory treatments. “I so much want to live,” she said.

Typically absent from the claims about many “alternative treatments” are their risks. The significant harms that they can pose form the fabric of Paul Offit’s important and timely book. Offit writes in a lucid and flowing style, and grounds a wealth of information within forceful and vivid narratives. This makes his argument—that we should be guided by science—accessible to a wide audience.

Although conventional therapies can be disappointing, alternative therapies shouldn’t be given a free pass.... All therapies should be held to the same high standard of proof; otherwise we’ll continue to be hoodwinked by healers who ask us to believe in them rather than in the science that fails to support their claims. And it’ll happen when we’re most vulnerable, most willing to spend whatever it takes for the promise of a cure.

Offit is a pugilist in the battle against charlatanism, and he lands punch after punch with hard facts about harms.

The possibility of harm caused by natural products ... isn’t theoretical. Blue cohosh can cause heart failure; nutmeg can cause hallucinations; comfrey, kava, chaparral, Crotalaria, Senecio, jin bu huan, Usnea lichen, and valerian can cause hepatitis; monkshood and plantain can cause heart arrhythmias; wormwood can cause seizures; stevia leaves can decrease fertility; concentrated green tea extracts can damage the liver; milkweed seed oil and bitter orange (Citrus aurantium) can cause heart damage; thujone can cause neurological damage; and concentrated garlic can cause bleeding....And it’s not just the supplements themselves that might be harmful, but what’s contaminating them. In 2004, researchers at Harvard Medical School tested Indian (Ayurvedic) remedies obtained from shops near Boston’s City Hall. They found that 20 percent contained potentially harmful levels of lead, mercury, and arsenic.... These problems aren’t rare. Between 1983 and 2004, poison-control centers in the United States received 1.3 million reports of adverse reactions to vitamins, minerals, and dietary supplements, of which 175,268 required treatment in hospitals and 139 resulted in death. In 2012, the FDA [Food and Drug Administration] estimated that approximately 50,000 adverse reactions to supplements occurred every year.

A pediatrician expert in infectious diseases, Offit is particularly focused on vulnerable children, who do not have the agency to make choices for themselves. In such instances, deluded parents can prevent a child with a potentially curable malignancy from receiving proven therapies, while pursuing remedies that are not only nonsensical but also noxious. Early in the book we are introduced to Joey Hofbauer, a seven-year-old who, in 1977, developed a lump in his neck. He soon received a diagnosis of Hodgkin’s disease; his prognosis was excellent, with a 95 percent chance of complete remission: “Joey could live a long and fruitful life. But for Joey Hofbauer, the road to recovery wasn’t going to be easy.” The barriers in his path were not a lack of insurance or of access to expert oncologists. Rather, his parents, John and Mary Hofbauer, believed that there must be a more “natural” way to cure their son than toxic chemotherapy and radiation. They decamped to the Fairfield Medical Center in Montego Bay, Jamaica, to receive laetrile, a touted cancer “cure” extracted from apricot pits.