Semaglutide (GLP-1)

All three contain semaglutide but differ in dose, formulation, and FDA-approved indication. Ozempic (0.25–2 mg SC weekly) is approved for type 2 diabetes and cardiovascular risk reduction in T2D patients. Wegovy (0.25–2.4 mg SC weekly) is approved for chronic weight management in obesity or overweight with comorbidities. Rybelsus (3–14 mg oral daily) is approved for type 2 diabetes only—it uses SNAC (sodium N-[8-(2-hydroxybenzoyl)amino]caprylate) to enable oral absorption across the stomach lining, a pharmacological feat that distinguishes it from other GLP-1 agonists. The oral route achieves lower and more variable bioavailability (~1% vs. ~89% for SC) but provides a needle-free option.

Head-to-head data: The SURMOUNT-5 trial (2025) compared semaglutide 2.4 mg vs tirzepatide 10–15 mg in people with obesity—tirzepatide produced approximately 47% greater weight loss (20.2% vs 13.7% from baseline). Both are effective, but tirzepatide appears superior in direct comparison at maximum approved doses. Semaglutide has a longer clinical track record, more complete long-term safety data, and an approved oral formulation; tirzepatide has the superior efficacy profile. Choice between them involves insurance coverage, individual response, and side effect tolerance.

The SELECT trial (2023; PMID: 37964988) enrolled 17,604 adults with obesity/overweight without diabetes who had established cardiovascular disease. Semaglutide 2.4 mg weekly reduced major adverse cardiovascular events (MACE: CV death, MI, stroke) by 20% versus placebo over 3.3 years (HR 0.80, 95% CI 0.72–0.90). This is significant because: (1) it demonstrates cardioprotection independent of diabetes; (2) the population is the first large obese-but-not-diabetic group to show benefit from a GLP-1 agonist; (3) it expanded the approved indication for Wegovy to include CV risk reduction in the US.

GLP-1 receptors are expressed in the dorsal vagal complex (area postrema) in the brainstem—a region involved in nausea and vomiting control, and where the blood-brain barrier is incomplete. Semaglutide activation of these receptors is believed to cause nausea, vomiting, and decreased appetite simultaneously. GLP-1 also slows gastric emptying (by 25–40% at peak effect), which contributes to early satiety and post-meal nausea. The 4-week titration steps are designed to allow these receptors to partially accommodate the agonist, reducing nausea severity over time. Most patients experience peak nausea during the first 4–8 weeks, which resolves or becomes manageable with continued use.

Semaglutide for neurological conditions is an active research frontier. GLP-1 receptors are expressed in the brain's substantia nigra, hippocampus, and cortex. Retrospective data shows patients on semaglutide for diabetes have lower rates of Parkinson's disease onset. A Phase 2 trial (SPARK, NCT04269148) in Parkinson's disease completed in 2024 with results pending. For Alzheimer's, observational data suggests reduced dementia risk in GLP-1 agonist users, and Phase 3 trials are ongoing (EVOKE trial for semaglutide). For addiction, small studies show reduced alcohol and opioid cravings. None of these applications are approved as of 2026.

Key safety considerations from the FDA label and clinical experience: (1) Thyroid C-cell tumors: semaglutide causes C-cell hyperplasia in rodents at clinical exposures; relevance to humans is unknown but it is contraindicated in personal/family history of MTC or MEN2; (2) Pancreatitis: rare but potential risk; discontinue if suspected; (3) Gallbladder disease: rapid weight loss with semaglutide increases cholelithiasis risk; (4) Hypoglycemia: rare with semaglutide alone but significant risk when combined with insulin or sulfonylureas; (5) Gastroparesis: severe gastroparesis has been reported—the slowed gastric emptying mechanism can become pathological; (6) Suicidal ideation: FDA investigated but has not confirmed a causal link despite reports.

Semaglutide produces weight loss through multiple complementary mechanisms: (1) Central appetite suppression—GLP-1R activation in the hypothalamus (arcuate nucleus) and brainstem reduces the drive to eat by suppressing NPY/AgRP hunger neurons and activating POMC satiety neurons; (2) Reduced food reward—GLP-1R in the mesolimbic dopamine system reduces the pleasurable response to food, making highly palatable foods less reinforcing; (3) Delayed gastric emptying—slower stomach emptying prolongs satiety after meals; (4) Reduced caloric intake—all combined effects result in 20–35% spontaneous caloric reduction without deliberate dietary restriction. The average patient loses 15–17% body weight at 2.4 mg dosing without calorie counting.

Clinical trial data consistently shows significant weight regain after semaglutide discontinuation. The STEP 4 withdrawal trial (2021, N=803) showed that patients who stopped semaglutide after 20 weeks regained approximately two-thirds of their weight loss within 1 year, while those who continued maintained or increased their loss. This rebound occurs because semaglutide does not permanently reset the body's adiposity 'set point'—once the appetite suppression is removed, hunger returns to pre-treatment intensity. Most researchers now view obesity as a chronic disease requiring indefinite treatment, similar to hypertension or diabetes, rather than a condition where a finite treatment course produces lasting effects.