No medication in the history of modern weight loss has inspired as much awe as the latest class of obesity drugs. Wegovy and Zepbound are so effective that they are often likened to “magic” and “miracles.” Indeed, the weekly injections, which belong to a broader class known as GLP-1s, can lead to weight loss of 20 percent or more, fueling hype about a future in which many more millions of Americans take them. Major food companies including Nestlé and Conagra are considering tailoring their products to suit GLP-1 users. Underlying all this excitement is a huge assumption: They work for everyone.
But for a lot of people, they just don’t. Anita, who lives in Arizona, told me she “took it for granted” that she would lose weight on a GLP-1 drug because “the people around me who were on it were just dropping weight like mad.” Instead, she didn’t shed any pounds. Likewise, Kathryn, from Florida, hasn’t lost any weight since starting the medication in October. “I was really hoping this was something that would be a game changer for me, but it feels like it was just a lot of wasted money,” she told me. (I’m identifying both Anita and Kathryn by their first name only to allow them to speak openly about their health issues.)
Some people can’t tolerate the side effects of the drugs and have to stop taking them. Others simply don’t respond. For some, the strength of the dose, or length of the treatment, does not seem to make a difference. Appetites might remain robust; the “food chatter” in the brain may stay noisy. Together, both groups of less successful GLP-1 users account for a not-insignificant share of patients on these drugs—potentially up to a third. “We don’t really know why it happens, [but] we know it does happen,” Louis Aronne, an obesity-medicine specialist at Weill Cornell Medical College, told me. Despite the promise of a so-called Ozempic revolution, lots of “No-zempics” have been left behind.
Of the two biggest reasons some people don’t lose weight on GLP-1 drugs—side effects and nonresponse—the former is much more straightforward. The GLP-1 drugs Wegovy and Zepbound (which contain the active ingredients semaglutide and tirzepatide, respectively), are known for causing potentially gnarly gastrointestinal symptoms, such as nausea and vomiting, although most people’s reactions are mild and temporary. Yet some have it far worse. Severe, albeit uncommon, side effects include pancreatitis, severe gastrointestinal distress, low blood sugar, and even hair loss, which “can push people off” the drugs, Steven Heymsfield, a professor who studies obesity at Louisiana State University, told me. In one of the biggest studies of semaglutide, encompassing more than 17,000 people over about five years, nearly 17 percent of patients discontinued the medication because of side effects.
Far more mysterious are the people who tolerate the drugs but respond weakly to them—or sometimes not at all. Researchers have known this might happen since these drugs were in early clinical trials. About 14 percent of people who took semaglutide for obesity saw minimal impacts of less than 5 percent weight loss in one study, as did 9 to 15 percent of people who took tirzepatide in a similar one. In her own experience working with patients, “somewhere between a quarter and a third” are nonresponders, Fatima Cody Stanford, an obesity-medicine specialist at Harvard, told me, adding that it can take up to three months to determine whether the drug is working or not. That the same medication at the same dosage can lead to dramatic weight loss in one person and hardly any in another “remains confounding,” Aronne told me.
The broad explanation is that it has something to do with genetics. The drugs work by masquerading as the appetite-suppressing hormone GLP-1 and binding to its receptor, like a key fitting into a lock. Although the lock’s overall shape is generally consistent from person to person, its nooks and crannies can vary because of genetic differences. “For some people, that key just won’t fit right,” Eduardo Grunvald, an obesity-medicine doctor at UC San Diego Health, told me. In other cases, genes may limit the effects of these drugs after they bind to GLP-1 receptors. One possibility is that people metabolize the drugs differently: Some patients may break them down too quickly for them to take effect; others may process them too slowly, potentially building up such high levels of the medications that they become toxic, Heymsfield said.
For No-zempic patients, perhaps the most consequential impact of individual variation is on the propensity for obesity itself. “We are all very different from a genetic standpoint, in terms of our risk of weight gain,” Grunvald said. Numerous factors can drive obesity, including diet, environment, stress, and—most pertinent to GLP-1 drugs—altered brain function.
GLP-1 drugs target a pathway that regulates appetite and insulin levels. Some cases of obesity can be caused by a disruption in that particular mechanism, in which case GLP-1s can indeed be wondrous. But “not everyone has dysfunction in this particular pathway,” Stanford said. When that is the case, the drugs won’t be very effective. A different pathway, for example, controls the absorption of fat from food; another increases energy expenditure. In these people, GLP-1s might tamp down appetite to a degree, maybe leading to some weight loss, but a different drug may be required to treat obesity at its root. “It is not all about food intake,” Stanford said.
That’s not to say that No-zempics are out of options. They might have better success switching from one GLP-1 to the other, or even stacking them, Heymsfield said. Some patients who don’t respond to GLP-1s at all can get better results with older drugs that work on different obesity pathways, Aronne said. One, called Qysmia, a combination of the decades-old drugs phentermine and topiramate, can lead to an average weight loss of 14 percent body weight at its highest dose. If medications don’t work, bariatric surgery remains a powerful option, one that may even be growing in popularity. Last year, the number of bariatric surgeries performed in the U.S. grew despite the boom in GLP-1 usage, a trend that some expect to continue, because so many people don’t tolerate the drugs.
The intense hype around the game-changing nature of GLP-1s makes it easy to forget that they are, in fact, just drugs. “Every drug that’s ever been made” works in some people and not in others, Heymsfield said; there’s no reason to think GLP-1s would be any different. Remembering that they are in an early stage of development has a sobering effect. Eventually, obesity drugs may leave fewer people behind. The category is expanding rapidly: By one count, more than 90 new drug candidates are in development.
They are evolving to attack obesity from multiple fronts, which, at least in theory, widens their net of potential users. In an early study on an experimental candidate named retatrutide—called a triple agonist because it acts on GLP-1 as well as two other targets involved in obesity, GIP and glucagon receptors—100 percent of people on the highest dose lost 5 percent or more of their body weight. New candidates are also expected to have fewer side effects. They have to, Heymsfield said, because the competition is so steep that any new drug has to be “as good with less side effects, or better.”
But no matter how good these drugs get, it’s unrealistic to think that they’ll become a one-size-fits-all treatment for everyone with obesity. The disease is simply too complex, with too many drivers, for a single type of medication to treat it. More than 200 different drugs exist for treating high blood pressure alone; in comparison, Aronne said, regulating weight is “far more complicated.” The future, rife with options, holds promise that No-zempics may find a way forward. Yet considering all the unknowns about obesity and what causes it, that may not be enough to guarantee that they will see the results they want.