How GLP-1 Medications Work

"GLP-1" gets used as a catch-all these days, attached to weight-loss medications, telehealth ads, and product names that all sound alike. The term itself refers to a hormone the body already makes. The medications built around it are engineered copies of that hormone, designed to stay active in the body far longer than the original. Before looking into any specific program, it helps to know what these medications actually do and what their approval covers.

A hormone your body already makes

GLP-1 stands for glucagon-like peptide-1, a hormone the gut releases after eating. It prompts the pancreas to release insulin when blood sugar is rising, slows how quickly food leaves the stomach, and reaches receptors in brain areas involved in appetite. On its own, the hormone breaks down within minutes. GLP-1 receptor agonist medications, including semaglutide and tirzepatide, are built to activate those same receptors for a full week at a time.

Tirzepatide adds a second mechanism: it also activates GIP receptors, a related hormone pathway that semaglutide doesn't touch. How much each pathway contributes is still being worked out in mechanistic studies. What matters for a reader is that tirzepatide's trial results come from a different kind of molecule, so the two shouldn't be treated as interchangeable just because both get called "GLP-1" in casual conversation.

The label describes the same set of effects for the medication: it delays gastric emptying, stimulates insulin release in a glucose-dependent way, and decreases calorie intake, an effect the label says is likely mediated through appetite. The same prescribing information states the drug is approved for use alongside a reduced-calorie diet and increased physical activity, not as a standalone treatment.

Why the dose starts low and climbs slowly

These medications are injected once a week, and every approved schedule starts well below the eventual target dose. The current label for the branded semaglutide product sets out a stepped schedule: a low starting dose held for four weeks, then four-week steps upward, with the maintenance dose beginning at week 17. The amounts at each step are a prescriber's call, and the label is written for them.

That slow climb isn't incidental. The label says the escalation schedule is there to reduce the risk of gastrointestinal adverse reactions, and nausea, diarrhea, and vomiting sit near the top of its list of common adverse reactions. It also tells prescribers to consider delaying a step by four weeks when someone doesn't tolerate the current dose. In a telehealth program, that pacing runs through the provider, which is part of what the ongoing check-ins are for. The Embody GLP-1 program page describes its own intake, provider review, and messaging setup.

Who the approval actually covers

FDA approval for these medications isn't a general weight-loss endorsement. For weight reduction, the branded semaglutide label covers adults and patients aged 12 and older with obesity, plus adults with overweight who also have at least one weight-related condition. In the adult trial the label describes, that meant a BMI of 30 or higher, or 27 to 29.9 with a condition such as treated high blood pressure or sleep apnea.

The pivotal trial behind that approval, STEP 1, enrolled 1,961 adults who met those same criteria and specifically excluded anyone with diabetes. Over 68 weeks, the group taking semaglutide lost an average of 14.9% of body weight, compared with 2.4% in the placebo group, with both groups following the same diet and activity program.

Tirzepatide's SURMOUNT-1 trial used a similar design: adults with obesity or overweight, no diabetes, randomized to 5 mg, 10 mg, or 15 mg of tirzepatide or to placebo over 72 weeks. Average weight loss was 15.0% on the lowest dose and 20.9% on the highest, against 3.1% for placebo, counting everyone as randomized. Because tirzepatide works through two hormone pathways instead of one, its results aren't a direct stand-in for semaglutide's.

Both medications require a prescription. Nothing in this category is available over the counter, and a licensed provider is expected to review a person's health history and current medications before prescribing either one. That evaluation is also where the choice between the two medications, and between the label's two maintenance doses, actually gets made.

Compounded versions are a different product

Ads in this category sometimes offer "compounded" semaglutide or tirzepatide at a price below the brand-name version. Compounding lets a state-licensed pharmacy or an outsourcing facility prepare a version of a medication. FDA is blunt about what that produces: compounded drugs are not approved by FDA, and they do not go through the premarket review for safety, effectiveness, and quality that an approved drug does. That holds even when a licensed provider prescribes one after a real evaluation. FDA has also said the enforcement discretion that allowed wider compounding of these two drugs during the shortage has ended, now that neither appears on its shortage list. If a price looks unusually low, which product is actually being dispensed is a fair thing to ask.

Where to go from here

None of this replaces a conversation with a clinician about whether a GLP-1 medication fits a specific person's health history. What's covered here is the mechanism and the approval boundaries, the part that tends to get skipped in ads. The full guide collection walks through related ground, including how telehealth intake works and what trial data does and doesn't show, at the GLP-1 guide collection.

For anyone who already understands the mechanism and wants to see what an actual prescribing process looks like, the Embody GLP-1 program page walks through its intake and provider review.

Sources

• Wegovy (semaglutide) FDA prescribing information, revised 08/2025 (https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/215256s024lbl.pdf): approved population, dosing and titration schedule (4-step ramp to maintenance by week 17); regulatory source, current revision.
• Wilding et al., STEP 1, NEJM 2021, data verified via ACC.org trial summary (https://www.acc.org/latest-in-cardiology/clinical-trials/2021/02/18/19/23/step-1): 1,961 adults with obesity/overweight, no diabetes; semaglutide 2.4 mg weekly vs. placebo, both with lifestyle intervention, 68 weeks; mean weight change of 14.9% vs. 2.4% loss; limitation: excluded diabetes, self-selected volunteer sample.
• FDA statement, "FDA clarifies policies for compounders as national GLP-1 supply begins to stabilize" (https://www.fda.gov/drugs/drug-alerts-and-statements/fda-clarifies-policies-compounders-national-glp-1-supply-begins-stabilize): 503A pharmacies and 503B outsourcing facilities; states that compounded drugs are not FDA-approved and do not undergo FDA premarket review for safety, effectiveness and quality, and that semaglutide and tirzepatide no longer appear on FDA's drug shortage list; limitation: a shortage wind-down policy that can change, so the status should be rechecked before republishing.
• Jastreboff et al., SURMOUNT-1, NEJM 2022, data verified via peer-reviewed summary (https://pmc.ncbi.nlm.nih.gov/articles/PMC9606350): 2,539 adults with obesity/overweight, no diabetes; tirzepatide 5/10/15 mg weekly vs. placebo, 72 weeks; weight loss of 15.0% (5 mg), 19.5% (10 mg) and 20.9% (15 mg) vs. 3.1% for placebo, counting all randomized participants; limitation: dual GIP/GLP-1 mechanism differs from GLP-1-only drugs, so results aren't interchangeable across molecules.

Educational content only. Not medical advice, diagnosis, or treatment. Talk to a licensed clinician before starting, changing, or stopping any medication, peptide, or supplement.