Tirzepatide is a dual GIP and GLP-1 receptor agonist. The 72-week SURMOUNT-1 trial in adults with obesity produced the largest average weight reductions yet published for a weekly injectable. This guide covers those numbers, the population they came from, the side effects that come with them, and the regain problem nobody markets.
GLP-1 Therapy
Published by Astra, which offers some of the treatments discussed. Educational, not medical advice.
Tirzepatide is a once-weekly injectable peptide that activates two receptors: the glucose-dependent insulinotropic polypeptide receptor, usually written GIP, and the glucagon-like peptide-1 receptor, GLP-1. Semaglutide activates only the second. That dual action is the reason tirzepatide is described as a different class of drug rather than a stronger version of what came before.
Both GIP and GLP-1 are incretins, hormones your gut releases after eating that tell the pancreas to release insulin and tell the brain that the meal is happening. Tirzepatide is engineered to keep signalling on both channels for about a week per dose, which sustains effects on insulin secretion, gastric emptying, and appetite far beyond what natural incretins do.
Compounded tirzepatide, which is what many telehealth programs including Astra dispense, is prepared for an individual patient by a state-licensed US compounding pharmacy against a physician's prescription. Compounded preparations are not FDA-approved products. The branded product is FDA-approved for chronic weight management and for type 2 diabetes under separate brand names; a compounded version prescribed for an individual is a different regulatory category and should be described that way.
SURMOUNT-1 randomized 2,539 adults to weekly tirzepatide at 5mg, 10mg, or 15mg, or to placebo, for 72 weeks.1 Everyone in the trial also received lifestyle counselling, including a deficit diet and a physical activity target. Participants had a body mass index of 30 or higher, or 27 or higher with at least one weight-related complication. Critically, people with type 2 diabetes were excluded, so these results describe obesity treatment in people without diabetes.
Mean percentage weight change at 72 weeks was about 15 percent at 5mg, about 19.5 percent at 10mg, and about 21 percent at 15mg, against roughly 3 percent with placebo.1 Around 9 in 10 participants on tirzepatide reached at least 5 percent weight reduction. Roughly a third of those on the highest dose reached 25 percent or more, a threshold that had not previously been reached by a drug in a trial of this size.
Two details deserve emphasis because marketing tends to remove them. First, the average participant weighed around 105kg at baseline, so the percentages describe change from a high starting point. Second, the comparison group was not doing nothing: placebo participants got the same lifestyle program and lost about 3 percent. The drug effect is the gap between those, not the whole number.
The trial population also skewed toward women and toward participants in the United States, and it enrolled people willing and able to inject weekly and attend study visits for a year and a half. That is a motivated cohort. Real-world adherence is lower than trial adherence in essentially every chronic therapy ever studied, and weight outcomes track adherence closely.
The percentage figures tell you the outcome. A randomized 6-week phase 1 trial of ingestive behavior tells you the mechanism in humans.2 Participants with overweight or obesity attended laboratory test meals, and researchers measured how much they actually ate rather than asking them to recall it.
Participants on tirzepatide consumed on the order of 500 fewer calories at a test meal compared with placebo, along with changes in reported appetite and food preference.2 That is an unusually direct measurement. Most appetite research relies on self-reported hunger scales, which are noisy; a weighed test meal is a hard endpoint.
This matters for expectation-setting. Tirzepatide is not a metabolic accelerator. It reduces how much food you want and how much you eat, primarily through appetite signalling in the brain and slowed gastric emptying. People who expect the drug to work independently of what they eat are working from the wrong model, and the trials that produced the headline numbers all paired the drug with dietary counselling for exactly that reason.
For several years the comparison between tirzepatide and semaglutide had to be made across separate trials with different populations and protocols, which is a weak form of evidence. That changed when a randomized head-to-head trial of tirzepatide as compared with semaglutide for the treatment of obesity was published.3
The trial found greater average weight reduction with tirzepatide than with semaglutide in adults with obesity.3 That result is consistent with the indirect comparisons that preceded it, and a direct randomized comparison is a meaningfully higher grade of evidence than lining up two separate trials.
Greater average does not mean better for every individual. The distribution of response to both drugs is wide: some people respond strongly to semaglutide and poorly to tirzepatide, and tolerability differs person to person. Cost, supply, side-effect profile, and how a given body handles each molecule all belong in the decision. A trial reports a mean; a patient lives on the individual level.
The dominant adverse effects are gastrointestinal: nausea, diarrhea, vomiting, constipation, and dyspepsia. In SURMOUNT-1 these were mostly mild to moderate, mostly occurred during dose escalation, and were the leading reason participants discontinued.1 They are not rare. A meaningful fraction of people taking this drug feel notably unwell for stretches of the escalation period.
The standard mitigation is slow titration: start low, increase at intervals, and hold a dose rather than escalating through symptoms. This is a clinical decision, not a self-directed one, and it is one of the practical arguments for being prescribed and supervised rather than sourcing a vial and guessing.
Less common but more serious concerns include pancreatitis, gallbladder disease, and, in rodent studies, thyroid C-cell tumors, which is the basis of the contraindication in people with a personal or family history of medullary thyroid carcinoma or MEN2. Rapid weight loss of this magnitude also carries lean mass loss, which is why protein intake and resistance training are standard adjuncts rather than optional extras.
This is the part that gets left out of most consumer content, so we will state it plainly and without overclaiming. Weight regain after discontinuing incretin therapy is well documented across this drug class, in withdrawal-phase trial data and in real-world follow-up. People who stop generally regain a substantial share of what they lost, and appetite tends to return before the weight does.
The mechanistic reading is straightforward: the drug suppresses appetite while present and stops suppressing it when absent. Nothing in the mechanism is designed to reset a set point permanently. Obesity behaves like a chronic condition under treatment, and stopping treatment for a chronic condition generally returns the condition.
The honest planning question is therefore not what will I lose, but what does this look like at month 24 and beyond. Some people continue at a maintenance dose. Some taper with an intensive nutrition and strength program in place. Some regain. Anyone selling a 12-week course as a permanent fix is selling something the evidence does not support.
The trial results are real and they are large. They are also results from people who kept injecting for 72 weeks, with a dietitian, inside a study. That is the context the number belongs to.
Astra Editorial, reviewing the tirzepatide literature
No. Compounded tirzepatide is prepared for an individual patient by a state-licensed US compounding pharmacy on a physician's prescription. Compounded preparations are not FDA-approved products, though the active molecule is the same one studied in the published trials.
72 weeks of continuous weekly dosing alongside lifestyle counselling in SURMOUNT-1.
A randomized head-to-head trial found greater average weight reduction with tirzepatide in adults with obesity. Average is not individual: response and tolerability vary widely, and the right choice depends on your clinical picture.
Regain after discontinuation is well documented across this drug class. Appetite typically returns first. Planning for maintenance is part of the treatment decision, not an afterthought.
Compounded tirzepatide — online evaluation. Only charged if prescribed. Astra's program is prescribed and supervised by a licensed physician and dispensed by a state-licensed US compounding pharmacy, with titration guidance built into the plan.
This guide is educational and is not medical advice. Compounded medications are not FDA-approved. Speak with a licensed physician about your own care.