GLP-1 Receptor Agonists: Research & Context

Research overview

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted by intestinal L-cells in response to food intake. Its primary physiological roles include stimulating glucose-dependent insulin secretion, suppressing glucagon release, slowing gastric emptying, and signaling satiety to the hypothalamus. GLP-1 receptor agonists (GLP-1 RAs) are synthetic peptide analogs engineered to activate these same receptors with greater potency and significantly longer half-lives than endogenous GLP-1, which is cleared from circulation within minutes.

Semaglutide, a GLP-1 RA developed by Novo Nordisk, received FDA approval for type 2 diabetes management (Ozempic, 2017) and chronic weight management in adults with obesity or overweight and at least one weight-related comorbidity (Wegovy, 2021). Large cardiovascular outcomes trials — including SUSTAIN-6 and SELECT — established that semaglutide reduces major adverse cardiovascular events in appropriate patient populations. Tirzepatide, developed by Eli Lilly, is a dual GIP/GLP-1 receptor agonist approved under the brand names Mounjaro (type 2 diabetes, 2022) and Zepbound (chronic weight management, 2023). The dual-receptor mechanism appears to produce more pronounced effects on body weight and glycemic control than single-receptor agents in head-to-head research.

The period roughly spanning 2022 to 2025 saw extraordinary demand for both semaglutide and tirzepatide outpace the branded manufacturers' production capacity, prompting the FDA to add these drugs to its drug shortage list. Under sections 503A and 503B of the Federal Food, Drug, and Cosmetic Act, licensed pharmacies and outsourcing facilities may compound drugs that appear on FDA shortage lists under specific conditions. This provision led to a significant market for compounded semaglutide and tirzepatide. However, the compounded versions raise quality and safety questions the FDA has consistently flagged: compounded drugs do not undergo the same pre-market review for safety, effectiveness, and manufacturing quality as FDA-approved drugs.

The FDA declared the tirzepatide shortage resolved in October 2024 and the semaglutide shortage resolved in February 2025. Following shortage resolution, the agency set enforcement discretion timelines: state-licensed 503A pharmacies compounding semaglutide were required to cease by late April 2025; 503B outsourcing facilities received a discretion period through May 22, 2025. The FDA simultaneously proposed to exclude semaglutide, tirzepatide, and liraglutide from the 503B bulk substances list, a move the agency stated reflects its finding that there is no clinical need for outsourcing facilities to compound these drugs from bulk substances when the branded products are available. Litigation from the Outsourcing Facilities Association has challenged portions of these determinations, and the regulatory landscape continues to evolve.

Mechanism, in plain language

GLP-1 receptor agonists bind and activate GLP-1 receptors expressed throughout the body — in pancreatic beta cells, the gastrointestinal tract, the heart, and the central nervous system, particularly the hypothalamus and brainstem. In the pancreas, receptor activation enhances insulin secretion in a glucose-dependent fashion (meaning it does not cause hypoglycemia when glucose is normal). Simultaneously, it suppresses the glucagon secretion that would otherwise raise blood sugar. In the gut, GLP-1 RAs slow gastric emptying, blunting post-meal glucose spikes. In the brain, they modulate appetite circuits, reducing caloric intake. Tirzepatide additionally activates receptors for glucose-dependent insulinotropic polypeptide (GIP), an incretin that works synergistically with GLP-1 pathways on fat tissue and central satiety signaling.

What has been studied

Type 2 diabetes glycemic control (multiple Phase 3 randomized controlled trials)
Chronic weight management in adults with obesity (SURMOUNT and STEP trial programs)
Major adverse cardiovascular event reduction (SELECT trial for semaglutide)
Non-alcoholic steatohepatitis (NASH) — NASH/MASH clinical trial programs ongoing
Heart failure with preserved ejection fraction (STEP-HFpEF trial)
Addiction and substance use — exploratory and early-phase research

Regulatory context

Semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro/Zepbound) are FDA-approved drugs subject to standard post-market surveillance. During documented shortages (2022–2025), compounding under 503A and 503B was legally permissible under specific conditions. The FDA has expressed concern about unapproved compounded versions, citing instances of fraudulent labeling, incorrect dosing, and adverse event reports associated with products obtained outside licensed clinical settings. With the shortage now resolved, the window for lawful compounding of these molecules from bulk substances has closed for most providers. Any clinician or compounder still operating under the compounding model should verify current regulatory status, as FDA enforcement and ongoing litigation continue to shape what is permissible.

Considerations

GLP-1 receptor agonists are prescription medications with meaningful adverse effect profiles that require clinician supervision. Common effects studied in trials include nausea, vomiting, diarrhea, and constipation, particularly during dose escalation. Rare but serious effects documented in post-market surveillance include pancreatitis, gallbladder disease, and — in rodent studies — thyroid C-cell tumors (the clinical relevance of this finding in humans remains studied but unresolved for some agents). Cardiovascular effects require monitoring in certain patients. The compounded semaglutide and tirzepatide space has generated additional safety concerns related to inconsistent formulations, concentration errors, and unlicensed sources. These medications should only be used under the supervision of a licensed clinician who can conduct appropriate patient selection, baseline evaluation, and ongoing monitoring.