What Is a GLP-1? A Plain-English Explainer

GLP-1 receptor agonists are a class of medications that mimic a natural gut hormone called GLP-1 (glucagon-like peptide-1). They reduce appetite, slow the rate food leaves the stomach, and help regulate blood sugar. The class includes semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound). They are used for type 2 diabetes management and, at higher doses, for chronic weight management.

What does a GLP-1 receptor agonist actually do?

GLP-1 is a hormone your gut produces naturally every time you eat. It does several things at once:

• Signals fullness to the brain. GLP-1 crosses the blood-brain barrier and acts on appetite-regulating areas, telling you you've had enough to eat. The effect is meaningful — people on GLP-1 medications typically experience a significant reduction in hunger and food cravings.
• Slows gastric emptying. Food moves more slowly from the stomach into the small intestine. This keeps you feeling fuller for longer after a meal and blunts the post-meal blood sugar spike.
• Stimulates insulin release. GLP-1 signals the pancreas to release insulin when blood sugar rises — but only when it's actually elevated, not at baseline. This glucose-dependent mechanism is why these drugs have a lower hypoglycaemia risk than some older diabetes medications.

GLP-1 receptor agonist medications work by binding to the same receptors as the natural hormone — but they're engineered to last far longer in the body. Natural GLP-1 degrades within minutes. Semaglutide, for example, is designed to last a week, which is why it's injected once weekly.

Which medications are GLP-1 receptor agonists?

The medications most commonly referred to as "GLP-1s" include:

• Semaglutide — approved as Ozempic (injectable, type 2 diabetes), Wegovy (injectable, weight management), and Rybelsus (oral tablet, type 2 diabetes). See: What Is Semaglutide?
• Tirzepatide — approved as Mounjaro (type 2 diabetes) and Zepbound (weight management and sleep apnea). Technically a dual GIP/GLP-1 agonist — it activates both the GLP-1 receptor and the GIP receptor. See: What Is Tirzepatide?
• Liraglutide — an older GLP-1 agonist, sold as Victoza (diabetes) and Saxenda (weight loss). Daily injection; less commonly prescribed now that once-weekly options are available.

People often ask how tirzepatide and semaglutide compare. The key difference is the dual mechanism: tirzepatide also targets the GIP receptor, which is associated with greater average weight loss in head-to-head trials. See: Tirzepatide vs Semaglutide for Weight Loss.

What are GLP-1s approved for?

The approved indications vary by drug and brand, but GLP-1 receptor agonists are generally approved for:

• Type 2 diabetes management — to improve blood sugar control, often alongside other medications
• Chronic weight management — for adults with a BMI of 30+, or 27+ with at least one weight-related health condition
• Cardiovascular risk reduction — semaglutide has a specific cardiovascular outcome approval in people with type 2 diabetes and established cardiovascular disease
• MASH with liver fibrosis — semaglutide 2.4mg (Wegovy) received FDA approval for this indication in 2025
• Obstructive sleep apnea — tirzepatide (Zepbound) was approved for moderate-to-severe OSA in adults with obesity in December 2024

Prescribing decisions are made by clinicians based on your individual medical situation. The approved indication for a given brand doesn't mean that's the only reason a clinician might consider it.

Are GLP-1s safe?

GLP-1 receptor agonists have been studied in large clinical trials involving tens of thousands of participants. They are approved by major regulatory agencies including the FDA and EMA. That said, they are prescription medications with real side effects — and "safe" always means safe for a particular person in a particular clinical context.

The most common side effects are gastrointestinal: nausea, vomiting, diarrhoea, and constipation. These are most pronounced at the start of treatment and during dose increases, and they tend to improve over time. Starting at a low dose and titrating slowly minimises the likelihood of severe side effects.

Less common but more serious effects — including pancreatitis and gallbladder problems — are rare but documented. There are specific contraindications including personal or family history of medullary thyroid cancer or multiple endocrine neoplasia syndrome type 2.

Whether a GLP-1 is appropriate for you requires a full assessment of your medical history. That conversation belongs with your prescribing clinician — not an article.

What about muscle loss?

This is one of the most important and underappreciated aspects of GLP-1 use. GLP-1 medications work primarily by reducing appetite and food intake. That calorie reduction drives weight loss — but the body doesn't only burn fat when intake falls sharply. Without adequate protein and resistance exercise, a significant share of the weight lost can be lean muscle rather than fat.

This is true on any GLP-1 medication. The solution is the same regardless of which drug you take: hit your protein target, do resistance training, and consider creatine. For the nutrition angle on common side effects and nutrient gaps, see: Ozempic Side Effects: The Nutrition Angle. For the full muscle-protection picture, see: How to Keep Muscle While Losing Weight on a GLP-1.

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