Weight-loss drugs surged back into headlines this week after Eli Lilly—the maker of Mounjaro and Zepbound—became the first healthcare company ever to reach a trillion-dollar valuation. The explosive success of its GLP-1 agonists, originally developed for diabetes and now prescribed widely for obesity, has fueled unprecedented revenue and global demand. Their influence extends far beyond weight control: some GLP-1 drugs have been approved to reduce heart attack and stroke risk, and early research has raised hopes that they might treat neurological or even substance use disorders.
But as MIT Technology Review reports, the excitement is colliding with new evidence that underscores how little we truly know about these drugs. In the same week that Lilly hit its historic milestone, several studies delivered more sobering news—particularly concerning Alzheimer’s, pregnancy, and what happens when people stop taking GLP-1 medications.
Glucagon-like peptide-1 is a gut-derived hormone that regulates blood sugar. GLP-1 agonists mimic this hormone’s effects, and several—semaglutide, tirzepatide, liraglutide, exenatide—are now household names under brands like Ozempic, Wegovy, and Saxenda. Beyond lowering glucose, the drugs powerfully suppress appetite, helping many people lose significant weight. Research has also linked them to unexpected mental-health benefits, reduced cravings for alcohol and nicotine, and increased neuron growth in animals.
Yet some of the latest findings complicate the narrative. Novo Nordisk recently tested an oral semaglutide in 3,808 people with early Alzheimer’s symptoms. The drug did not slow cognitive decline, a result that left researchers deflated. “It was kind of crushing,” says Daniel Drucker of the University of Toronto, who has spent decades studying GLP-1. Still, he isn’t ready to close the door: studies suggest GLP-1 reduces brain inflammation and improves neuronal communication, hinting that the drugs could benefit cognitively healthy people if given early enough.
A second set of studies raised new concerns about pregnancy. Current recommendations ask patients to discontinue GLP-1 drugs two months before conceiving, mainly because their effects during human pregnancy remain unknown. Researchers already knew that many users regain weight once they stop the drugs. A study published this week in JAMA found the same pattern in pregnant individuals: those who stopped GLP-1s before pregnancy gained about 3.3 kilograms more than those who had never taken them. They also showed slightly higher rates of gestational diabetes, hypertensive disorders, and preterm birth. Confusingly, another recent study found the opposite—reduced risks among those who had taken the drugs before pregnancy—highlighting how unsettled the science remains.
Meanwhile, a new Danish study shows that postpartum use is rising as people try to shed pregnancy weight. Drucker says he receives frequent questions about this trend, but warns that postpartum physiology is complex, involving dramatic hormonal changes, breastfeeding, bonding, and neurological shifts. Scientists have no clear understanding of how GLP-1 agonists might interact with any of it.
Another study added further uncertainty by examining what happens when patients stop tirzepatide after long-term use. Participants took the drug for 36 weeks; afterwards, half were switched to a placebo. Most of that placebo group regained over 25% of their lost weight within a year. A quarter regained more than 75%, and nearly 1 in 10 ended up heavier than before they began treatment. Heart-health improvements also deteriorated. These results raise difficult questions: Will many people need to take GLP-1 drugs indefinitely? Is lifelong use safe? And how should treatment be tailored by age, health status, or underlying conditions?
The unknowns don’t stop there. Researchers still don’t understand the long-term effects of GLP-1 drugs in children, how they influence substance use disorders, or what prolonged use means for healthy-weight people who take them solely for cosmetic weight loss. The drugs’ rapid cultural ascent has outpaced the science behind them.
Earlier this year, Drucker received the Breakthrough Prize in Life Sciences, and Hollywood celebrities eagerly thanked him for his work. Their gratitude left him uneasy. “A lot of these people don’t need to be on these medicines,” he says.
As GLP-1 drugs continue to reshape medicine, metabolism, and global markets, this week’s findings—spotlighted by MIT Technology Review—serve as a reminder that even the most promising breakthroughs carry shadows. For all their power, these drugs remain defined as much by what we don’t know as by what we do.
