Peptides, Explained · Part 2 of 5
The weight-loss shots
Semaglutide, tirzepatide and retatrutide: how they work, what the trials actually showed, and what they cost you.
Our full video overview of the peptide landscape. The weight-loss drugs covered in this article are the section with the strongest evidence behind them.
This is the one part of the peptide landscape where the evidence is not in dispute. These are approved medications with large clinical trials behind them, and they work by copying a hormone your own gut releases after you eat. They are also the reason the whole category is having a moment.
Proven Large human trials, FDA approval, and hard outcomes measured over years — including fewer heart attacks and strokes, not just lower numbers on a scale.
Semaglutide — Ozempic, Wegovy, Rybelsus
In plain English: a copy of GLP-1, a hormone your gut releases after a meal. It gets you full sooner and keeps you full longer, while improving how your body handles blood sugar.
What the evidence shows: in the main obesity trial, people lost about 15% of their body weight over roughly 15 months, compared with about 2% on a placebo. A separate large trial in people with existing heart disease found fewer heart attacks and strokes — the first weight-loss drug shown to do that.
Cells lining your gut sense food arriving and release GLP-1. It is meant to be a brief signal, so an enzyme in your blood called DPP-4 destroys it within about two minutes.
Semaglutide is that same message rebuilt to survive. The sequence is altered at the exact point the enzyme attacks, and a fatty acid chain is attached that clips onto albumin, the general-purpose transport protein in your blood. Riding on albumin, it circulates for about a week instead of two minutes — which is why it is a weekly injection rather than something you take after every meal.
Once circulating it acts in four places. In the pancreas it prompts insulin release, but only when blood sugar is already high, which is why on its own it rarely pushes blood sugar too low. Also in the pancreas, it stops the release of glucagon, the hormone that tells your liver to dump stored sugar into the blood. In the stomach, it slows the muscular pump at the exit so food leaves gradually. And in the brainstem and hypothalamus, it activates the circuits that produce the feeling of having eaten enough.
The catch: nausea, vomiting, diarrhea and constipation are common. Gallbladder problems, a pancreatitis warning, and kidney injury if vomiting leaves you dehydrated. Dangerously low blood sugar if combined with insulin. Because it slows the stomach, food can still be sitting there during anesthesia, which is a genuine surgical hazard — tell your surgeon well before any procedure. It carries the FDA's most serious warning label over a rare thyroid tumor seen in rodents, and it is off limits for anyone with medullary thyroid cancer or the MEN2 syndrome in the family.
Notice that the side effects are not separate from the benefits — they are the same mechanism. The lasting fullness and the nausea both come from the slowed stomach. This is the recurring lesson of the entire series: you usually cannot keep the effect you want and discard the one you don't, because they are the same lever.
It is also why these drugs are started at a low dose and increased slowly. The physician interviewed in our source material skipped ahead out of curiosity and spent the night violently ill. Escalating gradually is the whole point.
Tirzepatide — Mounjaro, Zepbound
In plain English: the same concept, but it presses two hormone buttons instead of one.
What the evidence shows: bigger numbers — up to about 21% of body weight over 72 weeks, versus about 3% on placebo. In December 2024 it also became the first drug ever approved for moderate-to-severe obstructive sleep apnea in adults with obesity.
GIP is the other hormone your gut releases when food arrives, from a different stretch of intestine than GLP-1. Adding it does three things.
In the pancreas, GIP amplifies the same insulin release GLP-1 is already triggering. In fat tissue, it appears to improve how fat cells take up and store fat safely, rather than letting it spill into the liver and muscle where it does metabolic damage. And in the brain, activating the GIP receptor seems to blunt nausea while still adding to appetite suppression.
That last point is the interesting one. It is why tirzepatide generally produces more weight loss than semaglutide without a proportional increase in vomiting — the second button partly cancels the worst side effect of the first.
The catch: the same family of problems as semaglutide, including the same thyroid warning and the same contraindications, and the same slow-escalation rule.
Retatrutide — not approved, still in trials
In plain English: three buttons instead of two. The extra one, glucagon, may increase the energy you burn.
What the evidence shows: the best results anyone has produced — about 24% average weight loss at 48 weeks. But that is mid-stage testing. As of mid-2026 it had not been submitted to the FDA for approval.
Glucagon is usually described as insulin's opposite: the hormone that tells your body to release stored energy. Deliberately switching it on in a weight-loss drug sounds backwards, and that is what makes retatrutide interesting.
Glucagon raises your resting energy expenditure — the calories you burn simply existing — and drives the liver to consume its own stored fat. So the three receptors divide the labor: GLP-1 reduces appetite, GIP strengthens the insulin response, and glucagon increases energy burned. Intake down and expenditure up at once, which no currently approved drug in this class does.
The catch is built into the same design. Glucagon's normal job includes raising blood sugar, so the drug depends on its own GLP-1 arm to hold that in check, and the dose-dependent rise in heart rate seen in trials most likely traces to the glucagon component too.
There is no legitimate way to buy retatrutide. Anyone taking it today is purchasing an unapproved drug from an unregulated seller — the highest-risk combination in this entire series. "Promising phase 2 results" and "proven safe over years" are very different statements.
One point that reframes everything
Orforglipron, approved in April 2026 for weight management, is a once-daily pill. It is not a peptide at all — and it presses exactly the same button as Ozempic.
There is a related detail that explains why this whole gray market exists. US law draws a line at 40 amino acids: a chain of 40 or fewer is regulated as an ordinary drug, while anything longer becomes a "biologic" under far stricter rules that compounding pharmacies cannot work around. Semaglutide is 31 amino acids. Tirzepatide and retatrutide are both 39. Sitting just under that line is precisely what makes compounded and copied versions of this class legally possible.
The honest summary
These drugs are the real thing. For someone who is metabolically unwell, they can create the conditions in which training, sleep and adequate protein finally start working. That is a genuine and underrated benefit.
They are not a body-recomposition shortcut for someone already lean, and the loss of muscle during rapid weight loss is a real problem — which is exactly where resistance training and protein intake stop being optional. If you take nothing else from this part: these work best as a medical tool that buys you the capacity to build the foundation, not as a replacement for it.
Educational information, not medical advice. Nothing here is a recommendation to use or avoid any compound, and no doses are given. Most compounds discussed in this series are not FDA-approved and have no established human dose. Anyone considering them should work with a qualified physician and a regulated pharmacy. Prototype Training Systems does not sell, supply, prescribe or administer any of these.
- What peptides actually are
- The weight-loss shots — you are here
- Growth hormone boosters
- BPC-157 and TB-500
- The rest of the landscape
The foundation is the part that works
Every honest read of this evidence lands in the same place: sleep, protein, consistent training and real bloodwork is where nearly all of the benefit lives. That is what we coach. Start with a free No Sweat Intro — a conversation about where you are and where you want to go.
Book a Free No Sweat IntroThis article was researched and drafted with the help of AI writing tools, then fact-checked against primary sources — published clinical trials, FDA labels and regulatory records. Where our source material contained errors, they were corrected against those sources rather than repeated.
