You've probably seen the word “peptide” next to Ozempic, Mounjaro and their cousins. It isn't marketing. It describes what these medicines are made of, and it explains a lot: why most of them are shots, why they last a week, and why the newest pill is different.
What is a peptide?
A peptide is a short chain of amino acids, typically 2 to 50, linked by chemical bonds called peptide bonds. A longer chain, 51 or more, is called a polypeptide, and proteins are made from one or more polypeptides (NHGRI). Picture a string of beads, where each bead is one amino acid.
Your gut makes its own GLP-1 after you eat. It's a peptide hormone about 30 amino acids long (Holst, 2007).
How close is semaglutide to your own GLP-1?
Very close. The Ozempic label calls semaglutide a GLP-1 analogue with 94% sequence homology to human GLP-1. In bead terms, almost every bead in the chain matches the one your gut makes. The label also says the peptide backbone is produced by yeast fermentation, and gives its molecular weight as 4113.58 g/mol.
What are the tweaks that make it last a week?
Your own active GLP-1 lasts under 2 minutes in the blood, because an enzyme called DPP-4 cuts it apart (Ahrén and Schmitz, 2004). Semaglutide is changed in two places so it lasts about a week instead:
- Position 8 is changed to protect it from DPP-4, like swapping one bead so the scissors can't grab it.
- Position 26 carries a spacer and a fatty acid (a C18 fatty di-acid). The fatty tail lets it hold on to albumin, a protein in your blood. The label says albumin binding is the main reason it lasts so long. Its half-life is about 1 week.
Is tirzepatide a peptide too?
Yes. The Mounjaro label says tirzepatide is based on the sequence of GIP, another gut hormone. It has changed building blocks at positions 2 and 13, and a fatty acid (a C20 fatty diacid) attached through a linker at position 20. Its molecular weight is 4813.53 Da, and its half-life is about 5 days.
Why are most GLP-1s shots instead of pills?
Your stomach is built to digest protein, and peptides are made of the same building blocks. A shot skips the gut. That makes a big difference in how much reaches your blood:
| Form | Amount absorbed |
|---|---|
| Semaglutide shot (Ozempic) | 89% |
| Rybelsus tablets | About 0.4% to 1% |
| Ozempic tablets | About 1% to 2% |
| Wegovy tablets | About 1% to 2% |
Semaglutide tablets contain a helper ingredient called SNAC (salcaprozate sodium) to get any of it absorbed (Wegovy label). The Rybelsus and Ozempic tablets label shows that how much water you take it with matters too: absorption was higher with a small sip than a full glass. That's why semaglutide pills come with strict rules about water and an empty stomach.
Why isn't Foundayo a peptide?
Foundayo (orforglipron) works on the same GLP-1 receptor, but it isn't a chain of amino acids. It's a small molecule: a compact chemical structure, not a string of beads. The Foundayo label gives its molecular weight as 902.0 g/mol, about a fifth of semaglutide's, and describes it by a chemical formula rather than an amino acid sequence. Its first big trials describe it as an oral small-molecule GLP-1 receptor agonist (Rosenstock et al., 2025).
The FDA approved Foundayo on April 1, 2026 (FDA). The label says about 77% was absorbed after a 0.8 mg dose, it peaks 4 to 8 hours after a dose, and its half-life is about 29 to 49 hours, which is why it's taken every day. Absorbing more doesn't mean it works better for any one person; it just explains why it's built as a pill.
How does your body break peptides down?
Mostly by taking it apart. The Ozempic label says semaglutide is broken down by cutting the peptide backbone and breaking down the fatty side chain, step by step. Only about 3% of a dose leaves in urine unchanged.
What about “research peptides” sold online?
These are not the same as prescription GLP-1s. The FDA says it has warned companies that illegally sold unapproved drugs containing semaglutide, tirzepatide and others, falsely labeled “for research purposes” or “not for human consumption,” and urges people not to buy them (FDA). The FDA also says some compounded semaglutide uses salt forms, like semaglutide sodium or semaglutide acetate, which are different active ingredients from the one in the approved medicines. If you have questions about a product you were offered, ask your pharmacist or prescriber.
How Checkin helps in your first weeks
Checkin is a free AI companion for GLP-1s that you text. For this part of life on a GLP-1, it does three things:
- Text “took my shot”
It's logged, so you can see your weeks add up.
- It checks in two days after your shot
A quick text to see how you're feeling, timed to your shot day.
- Ask anything
Plain answers from the FDA label, with a picture when it helps.
