GLP-1 Explained: Everything You Need to Know

What GLP-1 Actually Is

GLP-1 is a hormone, not a drug. Your intestines produce it naturally every time you eat — specifically from L-cells in the small intestine and colon. Under normal conditions, GLP-1 spikes briefly after a meal, triggers insulin release, slows digestion slightly, and then disappears within a few minutes.

The problem is that natural GLP-1 is too short-lived to produce meaningful weight loss. It's degraded almost instantly by an enzyme called DPP-4. So while you do get a small GLP-1 boost every time you eat, the effect is modest and transient.

GLP-1 medications are synthetic analogs engineered to resist DPP-4 degradation. They maintain elevated GLP-1 receptor activity continuously — for days at a time in the case of weekly injections — producing effects that natural GLP-1 never can.

The GLP-1 Medications Available Today

There are several approved GLP-1 receptor agonists. The most widely used for weight loss are:

Semaglutide — sold as Ozempic (diabetes, 0.5–2 mg) and Wegovy (obesity, 2.4 mg). Once-weekly subcutaneous injection. The STEP trial program showed 14.9% average weight loss.

Tirzepatide — sold as Mounjaro (diabetes) and Zepbound (obesity). A dual GIP/GLP-1 agonist, meaning it activates two receptors instead of one. The SURMOUNT-1 trial showed up to 20.9% weight loss, making it the most effective approved weight loss medication to date.

Liraglutide — sold as Victoza (diabetes) and Saxenda (obesity). Older, daily injection, less effective than semaglutide. Still prescribed but being displaced by weekly alternatives.

Oral semaglutide — sold as Rybelsus (diabetes) and in development for obesity. Less effective than injectable but preferred by some patients who avoid needles.

Who GLP-1 Medications Are For

FDA-approved indications are specific:

• Obesity medications (Wegovy, Zepbound) require BMI ≥30, or BMI ≥27 with at least one weight-related condition (high blood pressure, sleep apnea, type 2 diabetes, high cholesterol, or cardiovascular disease)
• Diabetes medications (Ozempic, Mounjaro) require a type 2 diabetes diagnosis

Off-label prescribing is widespread, particularly for people with BMI in the overweight range who don't meet strict obesity criteria. Telehealth platforms often use broader prescribing criteria.

What Results to Expect

Based on clinical trial data:

For a person starting at 220 lbs, this represents approximately 33 lbs lost on semaglutide or 46 lbs on tirzepatide at maximum efficacy.

Results come slowly. Most people lose 1–2 lbs per week in months 1–3, then the rate slows as the body adapts. Plateau is common at 12–18 months. Weight regain occurs in most patients who discontinue.

Side Effects to Know About

The most common side effects are gastrointestinal: nausea, vomiting, diarrhea, and constipation. These are most intense in the first 4–12 weeks as doses are titrated upward. Most patients experience nausea as mild to moderate; severe nausea affects roughly 10–15% of users.

A rare but serious concern is pancreatitis (pancreatic inflammation). GLP-1 medications are contraindicated in people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 (MEN2).

Bottom Line

GLP-1 medications work by sustaining the natural hunger-suppressing, digestion-slowing effects of a gut hormone that normally lasts only minutes. The clinical results — 15–21% weight loss — represent a genuine advance in obesity treatment. They're not magic, and they require ongoing use to maintain results, but for the right patient they're transformative.

Sources

• Wilding JPH et al., "Once-Weekly Semaglutide in Adults with Overweight or Obesity," NEJM, 2021
• Jastreboff AM et al., "Tirzepatide Once Weekly for the Treatment of Obesity," NEJM, 2022
• Lincoff AM et al., "Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes," NEJM, 2023