GLP-1 101: how semaglutide and tirzepatide work
Jillian Foglesong Stabile, MD
Board-certified Family Medicine · Diplomate, ABOM · Reviewed Jun 9, 2026
A hormone your gut makes after lunch turns out to be one of the most studied tools in modern obesity care. In a large trial published in 2021, adults taking weekly semaglutide lost, on average, about 15% of their body weight over roughly 16 months. Individual results vary, and a chart in a journal does not tell you whether the medicine is right for your body. But that one number changed how doctors talk about weight. Here is what that hormone is, and why it matters for you.
The short answer
GLP-1 medications copy a natural gut hormone that tells your brain you are full and slows how fast your stomach empties. Less hunger, smaller portions, steadier blood sugar. Semaglutide acts on one receptor; tirzepatide acts on two. Both are prescription medicines that need a physician.
What you'll learn
- What GLP-1 (and GIP) actually do in your body
- How semaglutide and tirzepatide differ
- What the clinical trials really showed (and what they did not promise)
- The side effects most people hit, and how doctors manage them
- Pills versus shots, including the 2026 oral options
- What these medicines cost, and how to read the price honestly
- Who is a candidate, and who is not
Chapter 01 · The science
The hormone behind the headlines
GLP-1 stands for glucagon-like peptide-1. It is a hormone your small intestine releases after you eat. It does a few quiet, useful things. It tells your pancreas to release insulin when blood sugar is high. It tells your liver to ease off making more sugar. And it slows how fast food leaves your stomach, so you feel full longer.1
Your own GLP-1 breaks down within minutes. That is the catch. The hormone works, but it does not last long enough to change your appetite over a whole day.
The medicines solve that. Semaglutide and tirzepatide are built to look like GLP-1 but resist the enzymes that would normally chop it up. So instead of a short pulse after a meal, you get a steady signal that lasts about a week from a single dose. The body reads it as "you have eaten; you are satisfied." Hunger drops. Portions shrink without a fight.4
They turn down the volume on appetite and on the food noise many people describe as constant. They do not speed you up.
Here is the one extra piece worth knowing. Tirzepatide does not copy just GLP-1. It also copies a second gut hormone called GIP (glucose-dependent insulinotropic polypeptide).5 GIP works alongside GLP-1 on blood sugar and, researchers think, on appetite and fat handling too. Hitting two receptors instead of one appears to add up to more than the sum of the parts.6
So in plain terms: semaglutide is a single-receptor medicine. Tirzepatide is a dual-receptor medicine. That difference shows up in the trial numbers, which we will get to.
Chapter 02 · The evidence
What the clinical trials actually showed
This is the section to read slowly, because the honest version matters more than the loud one.
For semaglutide, the STEP 1 trial enrolled 1,961 adults with obesity, or with overweight plus a related condition. Over 68 weeks (about 16 months), the group on weekly semaglutide 2.4 mg lost an average of about 14.9% of their body weight. The placebo group lost about 2.4%. Roughly 86% of people on semaglutide lost at least 5% of their weight.2
For tirzepatide, the SURMOUNT-1 trial ran 72 weeks. Average weight reduction came in around 16.0% on the 5 mg dose, 21.4% on 10 mg, and 22.5% on 15 mg, against about 2.4% for placebo.3, 7
14.9%
average weight loss on weekly semaglutide over 68 weeks (STEP 1)
22.5%
average on the top tirzepatide dose over 72 weeks (SURMOUNT-1)
Trial group averages, not promises. Individual results vary.
A few honest caveats, because the average is not a promise:
- Averages, not promises. These are group averages from controlled trials. Some people lose more, some lose less, and some do not respond well at all.
- Part of a plan. Trial participants also followed reduced-calorie eating and increased activity. The medicine was one part of a plan, not the whole plan.
- It settles. Weight loss generally slows and settles over time; it does not continue forever at the early pace.
No reputable source, and no good physician, will tell you that you will lose a specific number of pounds. The trials describe what happened, on average, to a defined group. Your body is its own study of one.
The short, fair answer on whether it stays off: for most people, the medicine is part of treating a chronic condition, not a short course. In follow-up studies, when people stopped, a meaningful portion of lost weight tended to return over the following year. That is not a failure of willpower. Obesity behaves like other chronic conditions; when you remove the treatment, the underlying biology often reasserts itself. This is exactly the kind of long-term question to work through with a physician, not a blog.
What would you actually pay?
30-second cost calculator
Chapter 03 · The experience
What to expect, week to week
You do not start at the full dose. Both medicines are titrated, meaning the physician starts you low and steps the dose up over weeks, usually in roughly four-week stages, until you reach a dose that works and that your body tolerates.3
The reason is comfort, not caution for its own sake. Going slowly gives your stomach time to adjust to the slower emptying, which keeps side effects manageable. The right dose is a clinical decision your physician makes with you; it is not something to self-adjust, and this article does not give dosing instructions.
Most people feel the first month as a softer appetite and earlier fullness. Meals that used to disappear now feel like too much halfway through. That is the point. The trade-off, especially early and right after each dose increase, is gastrointestinal. We will cover that next, because it is the part most people want straight talk on.
Chapter 04 · Straight talk
Side effects, told straight
The most common side effects of both medicines are gastrointestinal: nausea, vomiting, diarrhea, and constipation. They show up most during the dose-escalation phase, when your body is adjusting.1
To give you a sense of scale rather than a vague "some people": in the SURMOUNT-1 trial, up to about a third of people on tirzepatide (around 33%) reported nausea at some point. For semaglutide, nausea, vomiting, and diarrhea clustered in the days after a dose was raised, then eased.8, 2
The pattern matters more than any single number. For most people, GI symptoms peak in the first several weeks at a given dose, then improve a lot as the body adapts. Each time the dose goes up, a smaller version of that adjustment can repeat.1
This is where ongoing care earns its keep. Common, practical levers a physician may use include slowing the titration, holding a dose longer, or adjusting the plan if symptoms are stubborn. Everyday measures that often help: smaller meals, easing off very fatty or very large portions, and steady fluids.
There are also rarer but serious risks, including pancreatitis, gallbladder problems, and a boxed warning related to a specific type of thyroid tumor seen in animal studies. These are reasons the medicine is prescription-only and why your medical history matters. They are a conversation for your physician, who reviews your history before anything is prescribed. This article does not diagnose or recommend treatment.
The honest summary
Side effects are common, mostly digestive, mostly front-loaded, and mostly manageable with a physician who adjusts as you go. The ones that need watching are uncommon but real, which is exactly why a clinician stays in the loop.
Chapter 05 · Your options
Pills or shots? The 2026 picture
Yes, and 2026 changed the menu. For years the strongest GLP-1 medicines for weight were injections. Now there are real pill options.
- In December 2025, the FDA approved an oral semaglutide 25 mg tablet for chronic weight management, the first GLP-1 pill cleared for weight loss. In its supporting trial (OASIS 4), it produced about 13.6% mean weight loss at 64 weeks, with roughly a third of people on treatment reaching 20% or more.9, 10
- On April 1, 2026, the FDA approved orforglipron (brand name Foundayo™), a first-in-class oral small-molecule GLP-1, for adults with obesity, or overweight with a weight-related condition. Unusually, it can be taken without food or water restrictions. In the ATTAIN-1 trial, average weight loss ran about 7.8% on 6 mg, 9.3% on 12 mg, and 12.4% on 36 mg at 72 weeks.11, 12, 13
There is no single right answer, and the choice belongs to you and your physician. A few honest trade-offs:
- Strongest averages. Injections currently show the highest average weight loss in trials, especially dual-receptor tirzepatide.
- No needle. Pills remove the needle, which is the dealbreaker for some people, and oral options can be simpler to start.
- Daily vs weekly. The rhythm differs: most pills are daily; the leading injections are weekly.
Chapter 06 · The money
What it costs, honestly
Because there are really two prices, and people quote whichever one suits their argument.
The first is the list price, which is high. As of 2026, the branded injection list prices ran above $1,000 a month; one source put injectable Wegovy® near $1,349 and Zepbound® around $1,086 a month. Almost nobody pays the full list price.15
The second is the cash, or self-pay, price through manufacturer programs, which is much lower. In 2026, the oral semaglutide tablet launched around $149 a month for certain doses; manufacturer self-pay injection programs ranged roughly from the high-$100s to mid-$400s a month depending on product, dose, and subscription length.14, 16, 17
Prices are also moving. One manufacturer has signaled a roughly 50% cut to a branded list price starting in 2027. Treat any number you see, including the ones here, as a snapshot that can change.17
With one rule: ask for the full cost before you commit to anything. The hidden costs are usually not the medicine; they are the add-ons nobody named up front. Any honest provider, by law and by decency, should disclose every recurring charge before you are billed. A fair program tells you all four numbers:
- The medication price
- Any consultation or membership fee
- What happens at each dose change
- What you pay if you pause or stop
| Semaglutide | Tirzepatide | |
|---|---|---|
| Receptors | GLP-1 only | GLP-1 and GIP (dual) |
| Brand names (weight) | Wegovy® (injection); oral semaglutide tablet | Zepbound® (injection) |
| Trial average weight loss | ~14.9% over 68 wks (STEP 1) | ~16.0 to 22.5% over 72 wks by dose (SURMOUNT-1) |
| Common side effects | GI: nausea, vomiting, diarrhea, constipation | GI: nausea, vomiting, diarrhea, constipation |
| Typical dosing | Weekly injection; daily oral option | Weekly injection |
| Note | Physician decides dose | Physician decides dose |
Trial averages, not guarantees. Sources: STEP 1 (NEJM 2021); SURMOUNT-1 (NEJM 2022). Swipe to see the full table on a phone.
You will see "compounded" GLP-1 offers online, and the rules here have tightened. The FDA-approved, branded products (the ones in the trials above) are one category. Compounded medications are prepared by a pharmacy and are a separate category with different rules.
As of 2026, the large-scale supply shortages that once allowed wide compounding have largely resolved, and the FDA has moved to close broad compounding of these molecules. A narrow, individual pathway can still exist under Section 503A, but only with a specific, prescriber-documented clinical reason that the commercial product cannot meet a given patient's needs, tied to a patient-specific prescription. Lower cost, by itself, is not a qualifying reason.18, 19
One thing to know for your own safety: a compounded preparation is not a "generic" of a brand-name drug, and it has not gone through the same FDA approval as the branded product. It is a distinct medication; it is not FDA-approved and is not equivalent to the brand-name versions. If a clinical reason for compounding applies to you, that is a decision a licensed physician makes with a prescription written for you specifically.
Chapter 07 · The fit
Who these medicines are, and are not, for
In the trials and on the labels, these medicines are intended for adults with obesity, or for adults with overweight who also have a weight-related condition. Eligibility is a clinical judgment that weighs your full medical history, not a number you score yourself on.2
This is firmly a physician's call, but broadly, GLP-1 medicines are not for everyone. People who are pregnant or trying to become pregnant, people with certain thyroid cancer history, and people with some pancreatic or gallbladder conditions are among those who need careful evaluation or may not be candidates at all. The point of a real intake is to catch these before anything is prescribed. We do not diagnose or decide that in an article; a licensed physician does.
Many people who care about weight find that the right answer for them, today, is not a GLP-1 at all. That is a legitimate outcome of a good consultation.
Where to go from here
Not magic, not willpower in a pen. A studied medicine that works best inside a real plan.
- Know the landscape. You just did. Skim the sources below for the primary data.
- Check your fit. Take the 90-second quiz, or talk to a physician about your history and goals. No charge until your physician approves.
- Compare the whole package. Follow-up, labs, and every charge named up front, not just the headline percentage or price.
Take control of your health.
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- Same-day doctor visits and prescriptions
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Frequently asked questions
Sources
- Cleveland Clinic. "GLP-1 Agonists: What They Are, How They Work & Side Effects." Accessed May 2026. my.clevelandclinic.org
- Frontiers in Endocrinology. "Mechanisms of action and therapeutic applications of GLP-1 and dual GIP/GLP-1 receptor agonists." 2024. frontiersin.org
- GoodRx. "Tirzepatide's Mechanism of Action." Accessed May 2026. goodrx.com
- UAB News. "The GLP-1 revolution: What UAB researchers are discovering about how these drugs work." Accessed May 2026. uab.edu
- Wilding JPH, et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity" (STEP 1). New England Journal of Medicine, 2021. nejm.org
- Jastreboff AM, et al. "Tirzepatide Once Weekly for the Treatment of Obesity" (SURMOUNT-1). New England Journal of Medicine, 2022. nejm.org
- Eli Lilly and Company. "Lilly's SURMOUNT-1 results published in the New England Journal of Medicine." 2022. investor.lilly.com
- NCBI PMC. "Adverse Events Related to Tirzepatide." Accessed May 2026. ncbi.nlm.nih.gov
- HCPLive. "FDA Approves Semaglutide (Wegovy) Pill As First Oral GLP-1 for Weight Loss." December 2025. hcplive.com
- AJMC. "FDA Approves Oral Semaglutide as First GLP-1 Pill for Weight Loss." 2025 to 2026. ajmc.com
- Healio. "FDA approves Foundayo, an oral GLP-1, for adults with obesity." April 1, 2026. healio.com
- Pharmacy Times. "FDA Approves Orforglipron, First GLP-1 Pill Without Time, Food, or Water Restrictions." April 2026. pharmacytimes.com
- Drugs.com. "Foundayo (orforglipron) FDA Approval History." April 1, 2026. drugs.com
- Pharmacy Times. "From Rybelsus® to the Ozempic® Pill: The Next Generation of Oral Semaglutide." 2026. pharmacytimes.com
- Noom. "Wegovy vs. Zepbound: Real Monthly Costs With & Without Insurance (2026)." 2026. noom.com
- Eli Lilly. "Zepbound Cost Information." 2026. pricinginfo.lilly.com
- Fierce Pharma. "Novo unveils newly reduced self-pay prices for Wegovy, Ozempic after White House deal." 2026. fiercepharma.com
- Drug Topics. "GLP-1 No Longer on FDA's Drug Shortage List." 2025 to 2026. drugtopics.com
- Pharmacy Times. "FDA Moves to Permanently Close the Door on Compounded GLP-1s." 2026. pharmacytimes.com
- *This article is for general education and is not medical advice. It does not diagnose, prescribe, or recommend a dose. Talk with a licensed physician about your own health. Last reviewed: by, .*
Trademark attribution. Foundayo™ is a trademark of Eli Lilly and Company. Ozempic®, Rybelsus®, and Wegovy® are registered trademarks of Novo Nordisk A/S. Zepbound® is a registered trademark of Eli Lilly and Company. sipra is not affiliated with or endorsed by these companies.
Medically reviewed by
Jillian Foglesong Stabile, MD, FAAFP, DABOM
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