How Do GLP-1 Weight-Loss Medicines Work? (Ozempic, Wegovy, Mounjaro)

GLP-1 medicines don't “burn fat”. They change your appetite and digestion by copying a natural gut hormone. Here's the biology in plain language — and why tirzepatide adds a second mechanism.

What GLP-1 actually is

GLP-1 (glucagon-like peptide-1) is a hormone your gut releases when you eat. It's part of how your body signals “I've had enough” and manages blood sugar. The medicines everyone calls “weight-loss injections” are GLP-1 receptor agonists — molecules engineered to act like that hormone, but to last days rather than minutes, so one weekly (or daily) dose keeps the signal going.

The three main effects

Reduced appetite. They act on appetite centres in the brain, so you feel full sooner and stay full longer — you simply want less food.
Slower stomach emptying. Food leaves the stomach more slowly, which prolongs fullness (and is also why nausea is the most common side effect early on).
Steadier blood sugar. They boost insulin when blood sugar is high and curb glucagon — useful for diabetes, and part of why they were developed for it.

“Food noise” — the part people notice most

Many people describe a drop in “food noise”: the constant background chatter about snacks, cravings and the next meal. When that quietens, eating less stops feeling like a fight of willpower. That subjective change is often what makes the medicines feel different from past attempts at dieting.

Why Mounjaro adds a second mechanism

Tirzepatide (Mounjaro) is a dual agonist: as well as GLP-1, it activates the GIP receptor. Engaging both appears to enhance appetite suppression and metabolic effects, which is the leading explanation for its larger average weight loss compared with semaglutide. See semaglutide vs tirzepatide.

Why it's not magic

These medicines make eating less easier; they don't override the basics. Results depend on diet, activity and how long you stay on treatment, and most of the benefit fades if you stop — studies show meaningful weight regain after stopping. Protecting muscle with enough protein and some resistance exercise matters too (see diet & lifestyle). They also aren't for everyone — see am I eligible?

A registered provider can explain how this would apply to you

Frequently asked questions

Do GLP-1 injections burn fat directly?

No. They reduce appetite and slow digestion, so you eat less; weight loss follows from the resulting energy deficit, alongside diet and activity.

Why do they cause nausea?

Largely because they slow stomach emptying. It's usually worst when starting or increasing the dose and eases over time — see side effects.

Will the weight come back if I stop?

Often, partly — studies show meaningful regain after stopping, which is why these are usually viewed as long-term treatments alongside lifestyle change.

Sources & references

We cite primary sources and paraphrase them. Last reviewed June 2026. See our editorial policy and full sources hub.

1STEP programme — semaglutide for weight management — New England Journal of Medicine (Wilding et al., STEP 1, 2021). Average weight loss with semaglutide 2.4 mg (~15% at 68 weeks).

2SURMOUNT-1 — tirzepatide for weight management — New England Journal of Medicine (Jastreboff et al., 2022). Average weight loss with tirzepatide (up to ~21–22.5% at highest dose).

3Manufacturer Patient Information Leaflets (Novo Nordisk / Eli Lilly) — Novo Nordisk; Eli Lilly. Approved dosing, administration and side-effect information.

4SAHPRA — registered health products & safety alerts — South African Health Products Regulatory Authority. SA registration status of medicines and counterfeit / falsified-product warnings.

How GLP-1 weight-loss medicines work