“Peptide” is a chemistry word, not a therapeutic category. It means a short chain of amino acids — 2 to 50 of them — folded into a molecule your cells read as a signal. Insulin is a peptide. So is semaglutide. So are a dozen compounds sold online on the strength of a mechanism diagram and little else. The class spans one of the widest evidence gaps in medicine: at one end, peptide drugs tested in tens of thousands of randomized patients; at the other, peptides whose entire human dataset would fit on an index card. That range is not a reason to be cynical about the category — it is a reason to read it carefully. The practical skill is telling which end a given peptide sits at, and this guide commits to a position wherever the evidence lets it.
What a peptide actually is
A short chain of amino acids acts as a messenger. It binds a receptor and tells a cell to do one specific thing: secrete a hormone, open a channel, slow a process down. Your body makes thousands of them. A therapeutic peptide copies or blocks one of those signals on purpose.
Why the injection? That part is settled pharmacology. Your gut digests peptides into their component amino acids — the same way it handles the protein in a steak — so a peptide swallowed as a pill is mostly destroyed before it reaches the bloodstream. Oral semaglutide gets around this with an absorption enhancer and a large dose penalty, but it is the rare exception, not the pattern.
The peptides with the strongest evidence are the GLP-1 drugs
This is where the class earns its reputation, and it is worth being blunt about the asymmetry. The glucagon-like peptide-1 receptor agonists — semaglutide, and the dual agonist tirzepatide — carry some of the largest weight and cardiovascular trials ever run.
In STEP 1, 1,961 adults with obesity and no diabetes took semaglutide 2.4 mg weekly for 68 weeks and lost 14.9% of body weight, versus 2.4% on placebo. In SURMOUNT-1, 2,539 adults on tirzepatide lost 15.0%, 19.5%, and 20.9% at the 5, 10, and 15 mg doses over 72 weeks, against 3.1% on placebo. Those are among the largest average weight reductions a modern obesity drug trial has produced.
The benefit is not only on the scale. SELECT followed 17,604 adults with overweight or obesity and established cardiovascular disease but no diabetes; semaglutide 2.4 mg cut major adverse cardiovascular events — heart attack, stroke, cardiovascular death — by 20% in relative terms (hazard ratio 0.80). That is a hard-outcome trial, the kind most of the peptide market has never attempted. Those trials were run on the branded products, and their results belong to them. The compounded semaglutide and tirzepatide we prescribe are not FDA-approved and are not FDA-approved generics or equivalents of the drugs those studies tested.
Why they work — and why the nausea
The mechanism is settled, so here it is flatly. GLP-1 receptor agonists do three things. They slow gastric emptying, so food leaves the stomach more slowly and fullness lasts longer. They act on appetite circuits in the hypothalamus and hindbrain, lowering the drive to eat. And they improve glucose-dependent insulin secretion. Tirzepatide adds agonism at the GIP receptor, a second incretin pathway — the leading explanation for its larger effect in SURMOUNT-1, though exactly how much of the extra loss GIP accounts for is still being worked out.
The dose-response is visible in the SURMOUNT-1 numbers themselves: each step up in tirzepatide bought more weight loss. That is why the target dose matters, and why stopping short of it leaves results on the table.
The most common side effect follows directly from the first of those mechanisms. Nausea comes from delayed gastric emptying, which is why it clusters during dose escalation — in STEP 1 it was the most-reported adverse event and tracked the titration phase — and why it fades as the dose stabilizes. Because it follows mechanically from the drug’s action, it is predictable, and the dosing schedule is built around it.
Standard semaglutide titration starts at 0.25 mg weekly and steps up roughly every four weeks — 0.25 → 0.5 → 1.0 → 1.7 → 2.4 mg — slowing further if side effects flare. The starting dose is subtherapeutic on purpose. It is there to let the gut adapt, not to drive weight loss. Escalate too fast and you buy nausea you did not need to.
Stop the drug and the weight comes back
This is the part the marketing skips, and it is the single most important thing to understand before starting: GLP-1 weight loss is maintained by staying on the drug.
STEP 4 tested exactly this. Patients who stopped semaglutide at week 20 regained substantial weight over the following months, while those who continued kept losing. SURMOUNT-4 showed the same shape with tirzepatide: after 36 weeks on the drug, patients switched to placebo regained about 14% of body weight over the next year, while those who stayed on it lost a further 5.5%.
Our reading of that — labeled as clinical opinion, not trial data — is that these are treatments for a chronic condition, not a course of antibiotics. Discontinuing without a maintenance plan usually reverses the result. That is not a failure of the drug; it is what treating an appetite-regulation disorder looks like. If you and your clinician decide to come off, it should be a deliberate, supervised move toward a maintenance dose and a diet-and-training scaffold — not an abrupt stop.
The weight you lose includes muscle
Here the confidence has to drop, because the evidence is real but incomplete. DXA body-composition substudies of GLP-1 trials show that a meaningful fraction of the weight lost is lean mass, not only fat. This is not unique to these drugs — any rapid weight loss does it — but it matters more when the total loss is 15–20%.
The countermeasure is not exotic: resistance training plus enough protein, with clinicians commonly targeting roughly 1.2–1.6 g/kg/day. What we genuinely do not know yet is the long-term functional consequence — whether that lean-mass loss shows up years later as measurable differences in strength, mobility, or fracture risk. That data has not been generated at trial scale. Anyone who tells you the muscle question is settled, in either direction, is ahead of the evidence.
The other peptides — thinner ice
Not everything sold as a peptide has a file like this behind it, and we would rather say so than sell past it. The evidence for the GLP-1 drugs is, quite literally, orders of magnitude larger than for the peptides that follow.
Sermorelin is a growth-hormone-releasing hormone analog: it signals the pituitary to release its own growth hormone, preserving the natural feedback loop rather than overriding it the way injected recombinant HGH does. That mechanism is settled endocrine pharmacology. What is thin is the outcome evidence — there is nothing here resembling a STEP or SURMOUNT trial for the body-composition or anti-aging claims often attached to it. Reasonable physiology, limited hard endpoints.
Glutathione is the body’s main antioxidant tripeptide. NAD+ — a coenzyme, not actually a peptide, though it rides along in the same protocols — is required for mitochondrial energy production and DNA repair, and its levels fall with age. Both are physiologically real. Neither has the human-outcome trial base to carry the broad “detox,” “energy,” or “anti-aging” framing they are usually marketed under. If you use them, use them knowing the evidence is tier 3: plausible, mechanistically motivated, largely unproven.
That is the honest shape of this category — one corner holding some of the strongest drug-trial evidence in metabolic medicine, and several corners running on mechanism alone.
Who this isn’t for, and how we run it
If you have an active cancer, are pregnant or trying to be, or want a peptide to stand in for sleep, food, or training, this is the wrong tool. Peptides work alongside the boring basics, not instead of them. GLP-1 drugs also carry specific contraindications your clinician will screen for before prescribing — this is exactly the point where a real medical review matters, not a checkout page.
The compounded semaglutide and tirzepatide we work with are prepared by LegitScript-verified 503A US pharmacies under physician oversight, and the compounded preparations are not FDA-approved and are not FDA-approved generics or equivalents of the branded products. Care is cash-pay — no insurance required — and FSA/HSA eligible, and a US-licensed clinician reviews every case and sets the titration. Avoid gray-market “research only” peptide vials sold online; their sterility, supply chain, and dosing are all unverified. If you want to start, the intake is a clinical questionnaire, not a shopping cart.
The question we’re actually watching
Not whether these peptides take weight off — STEP 1, SURMOUNT-1, and SELECT settled that, at magnitudes no earlier drug class reached. The open question is what the 15–20% is made of over a decade. The lean-mass signal on DXA is real; the long-term functional readout — strength, mobility, and fracture risk in people who stay on these drugs for years — has not been run at trial scale. Whether resistance training and 1.2–1.6 g/kg of daily protein fully offset it, or only partly, is the trial we would most like to see. Until it reports, “protect your muscle while you lose the weight” is a clinical judgment call, not a proven protocol — and we treat it as one.
