GLP-1 for Obesity

Obesity is a chronic disease of excess body fat that damages health, and it is not rare: about 42% of US adults meet the clinical threshold of a BMI at or above 30, with about 1.1 billion people living with obesity worldwide. What sustains it is a persistent failure of energy balance regulation, shaped by genetics, hormones, sleep, stress, the food environment and income. That is why the World Health Organization treats it as a chronic disease needing ongoing medical care, not a lapse of discipline.

The downstream costs are broad. Carrying excess weight makes diabetes, heart disease, sleep apnea, fatty liver, worn joints and a range of cancers all more likely, shortens life expectancy and degrades quality of life. Treating it as a willpower problem is unkind, and it is the reason a treatable disease went undertreated for decades.

The physiology also explains why dieting so reliably fails over the long run. The body defends the weight it is at, adjusting appetite hormones, metabolic rate and reward signaling to pull intake back up. This class, together with the dual GIP-and-GLP-1 agonists that followed it, is the first set of medicines to work through those pathways, producing sustained loss at a scale that used to require surgery.

GLP-1 is a hormone the gut releases when you eat. It reaches the hypothalamus and turns appetite down, holds food in the stomach so fullness lasts, and quiets cravings. The drugs mimic that signal at levels well above what the body produces, which lowers intake without requiring deliberate restraint. That matters, because restraint is what fails over time. Tirzepatide also works on a second receptor, GIP, which appears to further influence how fat tissue is remodeled.

STEP-1 [1] enrolled 1,961 adults who had obesity, or excess weight plus a related condition, and none with diabetes. Over 68 weeks on weekly semaglutide 2.4 mg, mean loss reached 14.9% against 2.4% on placebo. The distribution matters more than the mean. 69.1% shed a tenth of their starting weight or more, half the arm reached 15%, and roughly a third got to a fifth, territory that had previously belonged to bariatric surgery.

SURMOUNT-1 [2] tested tirzepatide in 2,539 adults across 72 weeks. At 15 mg, mean loss was 20.9% against 3.1% on placebo, with 57% of that arm losing at least a fifth of their body weight. Zepbound was approved for chronic weight management in November 2023. Earlier, SCALE [3] had established liraglutide 3.0 mg as the first of this class approved for obesity, at a mean 8.4 kg against 2.8 kg over 56 weeks in 3,731 adults. Today that result is useful mainly as a marker of how far the class has moved.

Behavioral support adds to what the drug does. STEP-3 [4] paired semaglutide with a low-calorie diet and intensive counseling for a mean 16.0% loss, against 5.7% for placebo plus the same counseling. SURMOUNT-3 [5] is the cleaner demonstration: participants first lost an average 6.9% during a 12-week lifestyle lead-in, then received tirzepatide or placebo for 72 weeks. The drug arm lost a further 18.4% against 2.5% regained on placebo, 24.3% in total from the start of the lead-in. That is evidence the effects of the two interventions add together.

Both Wegovy and Zepbound carry an FDA approval for chronic weight management. It covers adults who have obesity, and adults who are overweight and also have at least one condition linked to their weight, for example high blood pressure, type 2 diabetes, abnormal lipids or sleep apnea. The labels print no BMI number in that indication; their trials enrolled at a BMI of 30 or above, or 27 or above with such a condition. Saxenda reads the same way. All three are licensed as adjuncts to a reduced-calorie diet and more activity. That wording describes how they were studied; it is not a formality.

Several things rule people out or warrant caution. The class carries a boxed warning concerning medullary thyroid carcinoma, and nobody whose own or family history includes that cancer, or multiple endocrine neoplasia type 2, should take one. Previous pancreatitis calls for care. None of these drugs is recommended in pregnancy. Anyone who also has type 2 diabetes may be given the diabetes-labeled versions of the same molecules instead, which address glucose and weight together.

Stomach side effects dominate: nausea, vomiting, diarrhea and constipation, worst while the dose is climbing and easing as it settles. Uncommon but serious events include acute pancreatitis, gallstones and a modest rise in resting heart rate. The thyroid warning rests on rodent findings. Human trials have so far not shown a rise in incidence, but they have not ruled one out either, which is why the contraindication stays.

Regain after stopping is the fact most often left out. The STEP-1 extension [6] followed participants for a year after semaglutide was withdrawn: they regained roughly two-thirds of what they had lost, and the cardiometabolic improvements largely went with it. That is what a chronic condition looks like when its treatment stops. It also means cost, coverage gaps and supply determine whether the result holds.