How do GLP-1 weight loss medicines actually work?

Key takeaways

• GLP-1 is a natural gut hormone; semaglutide and tirzepatide are longer-acting versions that last about a week.
• The main effect is on the brain's hunger and fullness signals, not on "burning fat".
• In large trials, people lost on average about 15% to 21% of body weight over 68 to 72 weeks, alongside diet and activity advice.
• Doses start low and rise slowly every four weeks to keep nausea manageable.
• The medicine works while you take it. A plan for protein, strength training and the eventual taper matters as much as the dose.

What is GLP-1, in plain words?

People come to me and say, "Doctor, just give me the injection." I'm happy to, if it suits you. But have you understood what it does in your body? Let me explain it the way I would across the desk.

GLP-1 stands for glucagon-like peptide-1. Cells in your small intestine release it when food arrives. Your own GLP-1 lasts only a few minutes in the blood before an enzyme breaks it down. That is why a normal meal makes you feel satisfied for a while, and then hunger comes back.

Medicines such as semaglutide are built to resist that breakdown, so one injection keeps working for about a week. Tirzepatide goes one step further: it acts on the GLP-1 receptor and also on the receptor for a second gut hormone called GIP. Both belong to a family doctors call "incretin" medicines.

What do these medicines actually do in the body?

Cleveland Clinic describes four main actions, and they explain almost everything my patients notice:

See, what happens is, most people who struggle with weight are not short of willpower. They are fighting a hunger signal that gets louder every time they diet. GLP-1 medicines turn that signal down. In practical terms, the tea-time biscuits stop calling to you, and one roti can feel like enough where three felt necessary before. That is the brain effect at work.

Because the insulin effect depends on blood sugar being high, these medicines on their own rarely cause low sugar. The risk rises if you also take insulin or sulfonylurea tablets for diabetes, which is why I want your full medicine list at the first consult.

How much weight do people lose, and how fast?

The best evidence comes from large randomised trials that also gave diet and activity counselling:

• Semaglutide 2.4 mg weekly (STEP 1, 1,961 adults): average loss of 14.9% of body weight at 68 weeks, compared with 2.4% on placebo. About 86% of people on the medicine lost at least 5%.
• Tirzepatide (SURMOUNT-1): average losses of 15%, 19.5% and 20.9% on the 5, 10 and 15 mg doses at 72 weeks, compared with 3.1% on placebo.

These are averages. Some people lose much more, some much less, and a few barely respond. I cannot tell you in advance which group you will be in, and anyone who does is guessing. Weight comes off gradually over many months, because the dose is raised slowly, usually every four weeks, from a small starting dose. Your weight at month two tells you very little about month twelve.

Why does the dose start so low?

The approved schedule for semaglutide for weight starts at 0.25 mg a week and steps up every four weeks to a maintenance dose, as listed in the US FDA prescribing information. The reason is the stomach. Slower emptying is useful, but if it happens too quickly you get nausea, burping, constipation or vomiting.

The label itself advises delaying the next step by four weeks if a dose is not tolerated. The dose is the skill. General protocols suit most people, but adjusting the pace for the individual, a shift worker, someone with acidity, a person on many medicines, is what keeps treatment comfortable. More on this in my guide to side effects and managing nausea.

Is it just eating less, or does something else happen?

Most of the weight loss comes from eating less, often without the constant hunger that makes ordinary diets collapse. That has two consequences you should understand:

1. You lose muscle as well as fat if you are not careful. In STEP 1, roughly 38% of the weight lost was lean mass. Eating enough protein and doing strength training is how you protect it. My guide on muscle loss on weight loss injections explains how.
2. Your body still wants the weight back. The medicine quiets hunger while you take it. If you stop abruptly, appetite usually returns. That is very important to understand before you start, not after. So I plan the stopping first. I work backwards from the result, the way I was taught chess: endgame first.

GLP-1 medicines also have effects beyond weight. Semaglutide has shown a reduction in heart attacks and strokes in adults with existing heart disease and overweight, which is now listed in its US label.

Who are these medicines for, and who should avoid them?

They are prescription medicines for adults with obesity, or overweight with a weight-related condition such as prediabetes, type 2 diabetes, high blood pressure, fatty liver or knee arthritis. For Indians, doctors increasingly use lower BMI and waist thresholds because we carry more risky fat at a lower weight (see why BMI cut-offs are lower for Indians).

They are not suitable if you are pregnant, trying to conceive or breastfeeding, if you or a close family member has had medullary thyroid cancer or MEN 2 syndrome, or if you have had a serious allergy to the medicine. A history of pancreatitis or gallbladder problems needs careful discussion. The NHS advises stopping semaglutide at least two months before trying for a baby.

Buying these medicines without a prescription, or from unverified online sellers, is a real risk. You cannot check the dose, the storage or what is inside the pen.

How I approach this at Dr Tvacha

I took GLP-1 treatment myself before I offered it to anyone. I do not like injections, honestly, but the needle is a fine insulin-type needle, and I understand the first weeks from the patient's side. In The Endgame Weight Programme, which runs online across India, we start with a Rs 499 video eligibility consult. Then I plan around the Triad: the right dose, enough protein and strength training. Every plan includes a written taper and maintenance plan from month one, and the decision to start is always yours.

Written by

Dr Amit KarkhanisAesthetic Physician · MBBS, Dip. Dermatology (UK) · Reg. No. 2001/02/1124

Founder of Dr Tvacha, practising in Thane since 2005. Leads the weight loss and hair programmes.