Best GLP-1 for Weight Loss: Should You Start With Semaglutide or Tirzepatide?

Every figure is an all-in monthly price covering medication, physician review, refill management and shipping, with no separate membership fee. GLP-1 prices are "from" prices because the vial, and the price, changes as the dose steps up. Current pricing always lives on each product page.

GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) are incretins: hormones the intestine releases when food arrives. Native GLP-1 is broken down within minutes. Semaglutide and tirzepatide are engineered to survive for days, bound to albumin in the blood, which is what makes once-weekly injection possible.

At the GLP-1 receptor, both medications:

• Act on appetite centres in the hypothalamus and brainstem, reducing hunger and increasing fullness. This is the effect patients describe as the noise around food going quiet, and it carries most of the weight effect.
• Slow gastric emptying, so fullness lasts longer. The same action explains the most common side effects.
• Release insulin only when glucose is high, and suppress glucagon, which is why neither causes meaningful low blood sugar on its own in people without diabetes.

Tirzepatide also activates the GIP receptor. GIP receptors sit on fat cells and in the brain as well as the pancreas, and the GIP arm is described as adding separate effects on insulin sensitivity and how fat tissue handles energy, and possibly on nausea tolerance. A 2022 study in The Lancet Diabetes & Endocrinology reported that tirzepatide improved insulin sensitivity by more than the weight change alone would predict. Some of the GIP story remains hypothesis.

Neither is a stimulant or a fat burner. The weight change in the trials follows from people eating less over a long period, alongside diet and activity changes.

The honest summary is that the two are more alike than different in everyday use: a weekly injection, a slow climb through strengths, and side effects that cluster around each step. The differences that matter for a first prescription are average effect size, outcomes data for particular conditions, and cost.

SURMOUNT-1 (New England Journal of Medicine, 2022) randomized 2,539 adults with obesity and no diabetes to tirzepatide 5, 10 or 15 mg weekly or placebo for 72 weeks, and reported substantial weight reduction at every dose against placebo. SURMOUNT-2 (The Lancet, 2023) ran the design in type 2 diabetes, where every agent in this class shows a smaller effect. SURMOUNT-3 (Nature Medicine, 2023) added tirzepatide after an intensive lifestyle programme and reported further reduction on top of it. SURMOUNT-OSA (New England Journal of Medicine, 2024) reported reduced obstructive sleep apnea severity in people with obesity.

SURMOUNT-5 (New England Journal of Medicine, 2025) is the direct comparison: 751 adults with obesity, or overweight with a weight-related condition, and no diabetes, randomized to the maximum tolerated dose of tirzepatide or semaglutide for 72 weeks. Weight and waist circumference fell more with tirzepatide on average, and more tirzepatide participants reached each weight-reduction threshold. Gastrointestinal events were the commonest in both arms, mostly mild to moderate and mostly during escalation.

What SURMOUNT-5 does not tell you

It was open-label, it compared the branded products rather than compounded preparations, and a difference in group averages says nothing about where any individual lands. The semaglutide arm still contained people who did better than the tirzepatide average. What a physician takes from it is that tirzepatide is the more potent agent on average, weighed against tolerability, history and cost for the person in front of them.

STEP 1 (New England Journal of Medicine, 2021) randomized 1,961 adults with overweight or obesity and no diabetes to semaglutide 2.4 mg weekly or placebo with lifestyle intervention for 68 weeks, with a large difference in favour of semaglutide. STEP 2 (The Lancet, 2021) ran the design in type 2 diabetes, STEP 5 (Nature Medicine, 2022) found the reduction held at two years, and STEP 8 (JAMA, 2022) put semaglutide ahead of daily liraglutide.

Beyond weight, SELECT (New England Journal of Medicine, 2023) enrolled more than 17,000 adults with established cardiovascular disease and overweight or obesity but no diabetes, and reported a 20% relative reduction in major adverse cardiovascular events. ESSENCE (New England Journal of Medicine, 2025) studied adults with metabolic dysfunction-associated steatohepatitis and moderate-to-advanced fibrosis and reported more resolution of steatohepatitis than placebo. Those are endpoints tirzepatide does not yet have equivalent placebo-controlled data for.

Every one of these trials ran the medication alongside a reduced-calorie diet and increased activity, and every one studied the branded, manufactured product.

Both start at a low strength that exists for tolerance rather than effect, and step up at roughly four-week intervals as tolerance allows. Four weeks is roughly four half-lives for both, and it maps onto the 28-day refill: each step up arrives as a new vial. These are the strengths Pepti supplies:

The practical difference: tirzepatide is drawn to the same 0.50 mL line at every strength, so when the dose changes the concentration changes and the line does not. Semaglutide's draw volume varies by strength, so the units for one strength are never the units for another. Whichever you are on, read each new label rather than carrying a number over, and never change a dose yourself; your physician sets your directions after reviewing your intake, and they are printed on the vial. Vial maths is explained in how long does a peptide vial last.

There is no prize for reaching the top of either ladder. The goal is the strength where appetite is quieter and portions are smaller without constant nausea.

The common side effects are the same family for both: nausea, early fullness, diarrhoea, vomiting, constipation and reflux, concentrated in the days after a dose increase and usually easing. In SURPASS-2, the trial that measured both side by side at diabetes doses, nausea, diarrhoea and vomiting rates were within a few percentage points of each other. Whether one feels heavier in practice depends far more on how fast the dose climbs than on which medication is in the vial. GLP-1 nausea: how to manage it covers what helps.

Less common but important for both: acute pancreatitis (severe, persistent abdominal pain is the reason to stop and call), gallbladder problems, and dehydration. Low blood glucose is rare on either alone but more likely with insulin or a sulfonylurea. Slower gastric emptying can change how oral medications, including oral contraceptives, are absorbed. Both approved labels carry a warning about thyroid C-cell tumours seen in rodents.

Compounding cost rises with the amount of active ingredient in the vial, and tirzepatide vials carry considerably more milligrams. At Pepti, semaglutide starts from $99 a month and tirzepatide from $159 a month, both rising as the strength steps up. Current figures by strength are on /cost/semaglutide and /cost/tirzepatide. Pepti is cash-pay and does not bill insurance.

When comparing, compare at the strength you are likely to need, not only at the starting strength. That is where cost most often decides the first prescription.

Most "peptides for weight loss" searches end at these two medications, because they are the only options in this category with large randomized trials. Everything else is adjunctive and does a different job: growth-hormone-axis peptides such as tesamorelin or CJC-1295 / Ipamorelin, metabolic peptides such as MOTS-c, and nutrient injections such as L-carnitine. No trial shows any of them adds weight loss on top of a GLP-1. The common pairings and what not to combine are in what peptides can you take with tirzepatide, which applies to semaglutide the same way; the wider category is in best peptides for weight loss.

Weight lost on a GLP-1 includes lean mass as well as fat. The countermeasures with the most behind them are a daily protein target and resistance training, which belong in the plan whichever medication you start on.

Semaglutide and tirzepatide are both active ingredients in FDA-approved branded medicines for type 2 diabetes and chronic weight management. The compounded vials Pepti supplies share the active ingredient, not the approval: compounded medications are not FDA approved. The FDA shortages ended for tirzepatide in December 2024 and for semaglutide in February 2025, and patient-specific 503A compounding continues where the prescriber documents a clinical reason for that patient. Pepti's tirzepatide contains glycine, an amino acid, in the 2.5 mg and 7.5 mg strengths, and no B12. More is in is compounded semaglutide safe and are peptides FDA approved.

These two medications have the strongest evidence base of anything prescribed for weight: large randomized trials and wide clinical use, for the branded products, run alongside diet and activity changes. Tirzepatide is the more potent agent on average in the one head-to-head obesity trial; semaglutide has more outcomes data in specific conditions and costs less. The withdrawal trials (STEP 4 and SURMOUNT-4) show that weight tends to return when either is stopped, which is why treatment is planned as long-term with 28-day refills; see what happens when you stop taking peptides. No honest provider can tell you how much weight you will lose or by when; trial averages say nothing about an individual.

Neither can be bought legitimately without a prescription. Sites selling them "for research use only" are not accountable for identity, purity, sterility or concentration. The prescription route:

1. Complete a free online intake covering your history, medications and what you want help with.
2. A physician licensed in your state reviews it, usually within 24 hours.
3. If it is appropriate, a PCAB-accredited, FDA-registered US 503A pharmacy compounds it to that prescription. Lot test results are published at /quality/lab-results.
4. It ships refrigerated with ice packs, with sterile syringes, alcohol prep pads and your directions printed on the vial.
5. Your physician stays reachable for questions and dose changes, with refills every 28 days, one vial per fill.

Step-by-step pages: how to get semaglutide prescribed online and how to get tirzepatide prescribed online.