GLP-1 Drugs for Weight Loss and Diabetes: How They Work

What GLP-1 receptor agonists are

GLP-1 (glucagon-like peptide-1) receptor agonists are a class of medications that mimic a hormone the small intestine naturally releases after a meal. That hormone signals the pancreas to release insulin, tells the brain that the stomach is full, and slows the movement of food through the gut. Pharmaceutical versions of this hormone—or molecules that act like it—are more potent and longer-lasting than the body's own supply 1Ref 1Collins L, Costello RA (2024).Glucagon-Like Peptide-1 Receptor Agonists.Mechanism of action of GLP-1 receptor agonists (glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying, hypothalamic appetite/satiety), contraindications (medullary thyroid carcinoma / MEN 2 boxed warning, pancreatitis), FDA-approved agents including semaglutide, liraglutide, and tirzepatide, and less-common adverse effects (mild tachycardia, gallbladder/biliary disease, acute kidney injury from volume contraction, injection-site reactions).

The class includes several distinct molecules. Semaglutide activates only the GLP-1 receptor. Tirzepatide activates both the GLP-1 receptor and a second receptor called GIP (glucose-dependent insulinotropic polypeptide), a dual action that appears to produce greater weight loss than GLP-1 activation alone 4Ref 4Jastreboff AM, Aronne LJ, Ahmad NN, et al.; SURMOUNT-1 Investigators (2022).Tirzepatide Once Weekly for the Treatment of Obesity.SURMOUNT-1 trial: tirzepatide 15 mg produced mean 20.9% weight loss at 72 weeks in 2,539 adults; 5 mg and 10 mg produced 15.0% and 19.5% respectively; placebo -3.1%; dual GIP/GLP-1 mechanism; common mild-to-moderate GI side effects during dose escalation. Liraglutide (Saxenda, Victoza) is an older daily-injection member of the class still in use, though it has been largely eclipsed by the once-weekly agents.

The same molecule can carry different brand names depending on its approved use. Semaglutide is sold as Ozempic (diabetes, once-weekly injection), Wegovy (weight management, once-weekly injection at a higher dose), and Rybelsus (diabetes, daily oral tablet). Tirzepatide is sold as Mounjaro (diabetes) and Zepbound (weight management) 1Ref 1Collins L, Costello RA (2024).Glucagon-Like Peptide-1 Receptor Agonists.Mechanism of action of GLP-1 receptor agonists (glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying, hypothalamic appetite/satiety), contraindications (medullary thyroid carcinoma / MEN 2 boxed warning, pancreatitis), FDA-approved agents including semaglutide, liraglutide, and tirzepatide, and less-common adverse effects (mild tachycardia, gallbladder/biliary disease, acute kidney injury from volume contraction, injection-site reactions).

How GLP-1 drugs work in the body

These medications act through three main pathways:

• Pancreas: GLP-1 agonists stimulate insulin secretion when blood glucose rises and suppress glucagon. Because both effects are glucose-dependent, the risk of hypoglycemia is lower than with older diabetes drugs—unless combined with insulin or sulfonylureas 1Ref 1Collins L, Costello RA (2024).Glucagon-Like Peptide-1 Receptor Agonists.Mechanism of action of GLP-1 receptor agonists (glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying, hypothalamic appetite/satiety), contraindications (medullary thyroid carcinoma / MEN 2 boxed warning, pancreatitis), FDA-approved agents including semaglutide, liraglutide, and tirzepatide, and less-common adverse effects (mild tachycardia, gallbladder/biliary disease, acute kidney injury from volume contraction, injection-site reactions).
• Brain: Hypothalamic GLP-1 receptors reduce appetite and increase satiety, driving sustained calorie reduction 1Ref 1Collins L, Costello RA (2024).Glucagon-Like Peptide-1 Receptor Agonists.Mechanism of action of GLP-1 receptor agonists (glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying, hypothalamic appetite/satiety), contraindications (medullary thyroid carcinoma / MEN 2 boxed warning, pancreatitis), FDA-approved agents including semaglutide, liraglutide, and tirzepatide, and less-common adverse effects (mild tachycardia, gallbladder/biliary disease, acute kidney injury from volume contraction, injection-site reactions).
• Gastrointestinal tract: Slowed gastric emptying extends fullness after meals and blunts post-meal blood sugar spikes 1Ref 1Collins L, Costello RA (2024).Glucagon-Like Peptide-1 Receptor Agonists.Mechanism of action of GLP-1 receptor agonists (glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying, hypothalamic appetite/satiety), contraindications (medullary thyroid carcinoma / MEN 2 boxed warning, pancreatitis), FDA-approved agents including semaglutide, liraglutide, and tirzepatide, and less-common adverse effects (mild tachycardia, gallbladder/biliary disease, acute kidney injury from volume contraction, injection-site reactions).

The combined result is lower caloric intake, improved glycemic control, and—in patients with established cardiovascular disease—evidence of reduced heart attack and stroke risk 3Ref 3Lincoff AM, Brown-Frandsen K, Colhoun HM, et al.; SELECT Trial Investigators (2023).Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes.SELECT trial: semaglutide 2.4 mg reduced major adverse cardiovascular events by 20% (HR 0.80, 95% CI 0.72-0.90) in 17,604 adults with preexisting CVD and obesity but without diabetes over mean 39.8 months; basis for Wegovy cardiovascular indication and Medicare coverage.

FDA-approved agents: what the evidence shows

Semaglutide 2.4 mg (Wegovy) is approved for chronic weight management in adults with a BMI of 30 or higher, or a BMI of 27 or higher with at least one weight-related condition such as hypertension, type 2 diabetes, or sleep apnea. In the STEP 1 trial—a 68-week randomized controlled study of 1,961 adults without diabetes—participants receiving semaglutide lost a mean of 14.9% of body weight, compared with 2.4% on placebo. Eighty-six percent of the semaglutide group achieved at least 5% weight loss 2Ref 2Wilding JPH, Batterham RL, Calanna S, et al.; STEP 1 Study Group (2021).Once-Weekly Semaglutide in Adults with Overweight or Obesity.STEP 1 trial: semaglutide 2.4 mg produced mean 14.9% weight loss vs. 2.4% placebo at 68 weeks in 1,961 adults; 86% achieved at least 5% weight loss; 4.5% discontinued due to GI events vs. 0.8% on placebo; FDA approval criteria for Wegovy (BMI ≥30, or ≥27 with comorbidity); black-box thyroid warning.

Tirzepatide 15 mg (Zepbound) received FDA approval for weight management in 2023. In the 72-week SURMOUNT-1 trial of 2,539 adults with obesity or overweight, the 15 mg dose produced a mean weight loss of 20.9%, with more than half of participants in the highest-dose group losing 20% or more of their starting weight—outcomes not previously seen with any approved weight-loss drug 4Ref 4Jastreboff AM, Aronne LJ, Ahmad NN, et al.; SURMOUNT-1 Investigators (2022).Tirzepatide Once Weekly for the Treatment of Obesity.SURMOUNT-1 trial: tirzepatide 15 mg produced mean 20.9% weight loss at 72 weeks in 2,539 adults; 5 mg and 10 mg produced 15.0% and 19.5% respectively; placebo -3.1%; dual GIP/GLP-1 mechanism; common mild-to-moderate GI side effects during dose escalation.

A 2025 head-to-head comparison (SURMOUNT-5) randomized adults with obesity to tirzepatide or semaglutide over 72 weeks: tirzepatide produced 20.2% weight loss vs. 13.7% with semaglutide, a 6.5 percentage-point difference 5Ref 5Aronne LJ, Horn DB, le Roux CW, et al.; SURMOUNT-5 Investigators (2025).Tirzepatide as Compared with Semaglutide for the Treatment of Obesity.SURMOUNT-5 head-to-head trial: at week 72, tirzepatide produced -20.2% (95% CI -21.4 to -19.1) mean body weight change vs. -13.7% (95% CI -14.9 to -12.6) with semaglutide 2.4 mg, a 6.5 percentage-point difference; tirzepatide superior for reduction in body weight and waist circumference.

Ozempic (semaglutide 0.5–2 mg) is FDA-approved for type 2 diabetes and, separately, to reduce the risk of major cardiovascular events in adults with type 2 diabetes and established heart disease. In the SELECT trial—17,604 adults with cardiovascular disease and obesity but without diabetes—semaglutide 2.4 mg reduced the composite endpoint of cardiovascular death, nonfatal heart attack, and nonfatal stroke by 20% over a mean follow-up of 39.8 months 3Ref 3Lincoff AM, Brown-Frandsen K, Colhoun HM, et al.; SELECT Trial Investigators (2023).Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes.SELECT trial: semaglutide 2.4 mg reduced major adverse cardiovascular events by 20% (HR 0.80, 95% CI 0.72-0.90) in 17,604 adults with preexisting CVD and obesity but without diabetes over mean 39.8 months; basis for Wegovy cardiovascular indication and Medicare coverage.

Liraglutide (Saxenda) is a daily injection approved for weight management and in use since 2014. In the 56-week SCALE Obesity and Prediabetes trial of 3,731 adults, liraglutide 3.0 mg produced a mean weight loss of about 8.0%, compared with 2.6% on placebo 6Ref 6Pi-Sunyer X, Astrup A, Fujioka K, et al.; SCALE Obesity and Prediabetes NN8022-1839 Study Group (2015).A Randomized, Controlled Trial of 3.0 mg of Liraglutide in Weight Management.SCALE Obesity and Prediabetes trial: liraglutide 3.0 mg produced mean -8.0% body weight change vs. -2.6% placebo at 56 weeks in 3,731 adults (a ~5.4 percentage-point difference); 63.2% on liraglutide vs. 27.1% on placebo lost at least 5% of body weight.

Who is and is not a candidate

Wegovy and Zepbound are indicated for adults with a BMI of 30 or higher, or a BMI of 27 or higher with at least one weight-related condition, used alongside a reduced-calorie diet and physical activity. Wegovy is also approved for adolescents aged 12 and older 2Ref 2Wilding JPH, Batterham RL, Calanna S, et al.; STEP 1 Study Group (2021).Once-Weekly Semaglutide in Adults with Overweight or Obesity.STEP 1 trial: semaglutide 2.4 mg produced mean 14.9% weight loss vs. 2.4% placebo at 68 weeks in 1,961 adults; 86% achieved at least 5% weight loss; 4.5% discontinued due to GI events vs. 0.8% on placebo; FDA approval criteria for Wegovy (BMI ≥30, or ≥27 with comorbidity); black-box thyroid warning.

These medications are contraindicated in patients with:

• Personal or family history of medullary thyroid carcinoma or MEN 2 syndrome (black-box warning; rodent studies showed thyroid C-cell tumors) 1Ref 1Collins L, Costello RA (2024).Glucagon-Like Peptide-1 Receptor Agonists.Mechanism of action of GLP-1 receptor agonists (glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying, hypothalamic appetite/satiety), contraindications (medullary thyroid carcinoma / MEN 2 boxed warning, pancreatitis), FDA-approved agents including semaglutide, liraglutide, and tirzepatide, and less-common adverse effects (mild tachycardia, gallbladder/biliary disease, acute kidney injury from volume contraction, injection-site reactions)2Ref 2Wilding JPH, Batterham RL, Calanna S, et al.; STEP 1 Study Group (2021).Once-Weekly Semaglutide in Adults with Overweight or Obesity.STEP 1 trial: semaglutide 2.4 mg produced mean 14.9% weight loss vs. 2.4% placebo at 68 weeks in 1,961 adults; 86% achieved at least 5% weight loss; 4.5% discontinued due to GI events vs. 0.8% on placebo; FDA approval criteria for Wegovy (BMI ≥30, or ≥27 with comorbidity); black-box thyroid warning
• History of pancreatitis
• Pregnancy
• Severe gastrointestinal disease 1Ref 1Collins L, Costello RA (2024).Glucagon-Like Peptide-1 Receptor Agonists.Mechanism of action of GLP-1 receptor agonists (glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying, hypothalamic appetite/satiety), contraindications (medullary thyroid carcinoma / MEN 2 boxed warning, pancreatitis), FDA-approved agents including semaglutide, liraglutide, and tirzepatide, and less-common adverse effects (mild tachycardia, gallbladder/biliary disease, acute kidney injury from volume contraction, injection-site reactions)

Patients with diabetic retinopathy may experience transient worsening with rapid blood sugar improvement. GLP-1 drugs are not indicated for people with a healthy weight.

Side effects and what to expect

Gastrointestinal effects are the most common reason people reduce the dose or discontinue treatment:

• Nausea — most frequent, especially in the first 4–8 weeks and after each dose increase; often improves as the body adjusts
• Diarrhea, vomiting, constipation — affect a meaningful minority; usually mild to moderate
• Abdominal pain — reported more often than with placebo in trials 2Ref 2Wilding JPH, Batterham RL, Calanna S, et al.; STEP 1 Study Group (2021).Once-Weekly Semaglutide in Adults with Overweight or Obesity.STEP 1 trial: semaglutide 2.4 mg produced mean 14.9% weight loss vs. 2.4% placebo at 68 weeks in 1,961 adults; 86% achieved at least 5% weight loss; 4.5% discontinued due to GI events vs. 0.8% on placebo; FDA approval criteria for Wegovy (BMI ≥30, or ≥27 with comorbidity); black-box thyroid warning4Ref 4Jastreboff AM, Aronne LJ, Ahmad NN, et al.; SURMOUNT-1 Investigators (2022).Tirzepatide Once Weekly for the Treatment of Obesity.SURMOUNT-1 trial: tirzepatide 15 mg produced mean 20.9% weight loss at 72 weeks in 2,539 adults; 5 mg and 10 mg produced 15.0% and 19.5% respectively; placebo -3.1%; dual GIP/GLP-1 mechanism; common mild-to-moderate GI side effects during dose escalation

In the STEP 1 trial, 4.5% of participants discontinued semaglutide due to gastrointestinal events, vs. 0.8% on placebo 2Ref 2Wilding JPH, Batterham RL, Calanna S, et al.; STEP 1 Study Group (2021).Once-Weekly Semaglutide in Adults with Overweight or Obesity.STEP 1 trial: semaglutide 2.4 mg produced mean 14.9% weight loss vs. 2.4% placebo at 68 weeks in 1,961 adults; 86% achieved at least 5% weight loss; 4.5% discontinued due to GI events vs. 0.8% on placebo; FDA approval criteria for Wegovy (BMI ≥30, or ≥27 with comorbidity); black-box thyroid warning. Eating smaller meals, avoiding high-fat foods, and staying well hydrated can reduce these effects during the titration period.

Less common but notable 1Ref 1Collins L, Costello RA (2024).Glucagon-Like Peptide-1 Receptor Agonists.Mechanism of action of GLP-1 receptor agonists (glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying, hypothalamic appetite/satiety), contraindications (medullary thyroid carcinoma / MEN 2 boxed warning, pancreatitis), FDA-approved agents including semaglutide, liraglutide, and tirzepatide, and less-common adverse effects (mild tachycardia, gallbladder/biliary disease, acute kidney injury from volume contraction, injection-site reactions): - A small increase in resting heart rate (mild tachycardia) - Gallbladder disease, including gallstones (faster weight loss increases gallstone risk) - Acute kidney injury, primarily from dehydration during severe GI illness - Injection-site reactions

Rare and serious: - Pancreatitis — seek care immediately for severe upper abdominal pain radiating to the back - Severe allergic reaction — rash, facial swelling, difficulty breathing

Long-term use note: The STEP 4 trial found that stopping semaglutide led to regain of most lost weight within one year, indicating ongoing use is needed to maintain benefit 7Ref 7Rubino DM, Greenway FL, Khalid U, et al.; STEP 4 Investigators (2021).Effect of Continued Weekly Subcutaneous Semaglutide vs Placebo on Weight Loss Maintenance in Adults With Overweight or Obesity: The STEP 4 Randomized Clinical Trial.STEP 4 discontinuation trial: after a 20-week run-in (mean 10.6% weight loss), switching to placebo led to weight regain (+6.9%) while continued semaglutide produced further loss (-7.9%) over the next 48 weeks, supporting that long-term use is necessary to maintain weight loss.

Cost, insurance, and access

Wegovy and Zepbound both list at roughly $1,000–$1,350 per month in the United States (est. list price; actual out-of-pocket cost varies by plan and savings programs). Coverage varies significantly:

• Commercial insurance often requires prior authorization with documented BMI thresholds; many plans exclude GLP-1s for weight loss even when covering Ozempic for diabetes.
• Medicare Part D covers Wegovy for patients with established cardiovascular disease under the SELECT indication 3Ref 3Lincoff AM, Brown-Frandsen K, Colhoun HM, et al.; SELECT Trial Investigators (2023).Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes.SELECT trial: semaglutide 2.4 mg reduced major adverse cardiovascular events by 20% (HR 0.80, 95% CI 0.72-0.90) in 17,604 adults with preexisting CVD and obesity but without diabetes over mean 39.8 months; basis for Wegovy cardiovascular indication and Medicare coverage, but not for weight loss alone without qualifying cardiovascular history.
• Medicaid coverage is inconsistent across states.
• Manufacturer savings cards (Novo Nordisk for Wegovy, Eli Lilly for Zepbound) can meaningfully reduce out-of-pocket costs for commercially insured patients. Patient assistance programs address lower income situations.

Primary care clinicians, endocrinologists, and obesity medicine specialists all prescribe these medications. Telehealth services can evaluate and prescribe in most states, often with shorter wait times than in-person specialty care.

Muscle loss on GLP-1 drugs

A recognized concern with GLP-1-driven weight loss is that a portion of the weight lost comes from lean muscle mass rather than fat alone. In the SURMOUNT-1 body-composition substudy, which used DXA scans in 160 participants, roughly a quarter (about 25%) of total weight lost with tirzepatide was lean mass and about 75% was fat—the same fat-to-lean ratio seen in the placebo group, a proportion typical of calorie restriction generally but larger in absolute terms given how much weight these drugs remove 8Ref 8Look M, Dunn JP, Kushner RF, et al. (2025).Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight.SURMOUNT-1 body-composition substudy (DXA in 160 participants): at week 72, tirzepatide reduced body weight -21.3%, fat mass -33.9%, and lean mass -10.9%; approximately 75% of weight lost was fat mass and 25% was lean mass (the same proportion as placebo).

Because GLP-1 agents suppress appetite broadly, reaching adequate protein intake requires deliberate effort. Clinicians increasingly discuss adequate protein intake and resistance training as protective strategies. For more detail, see the related page on [GLP-1 and muscle loss](/drug/glp-1-muscle-loss).