What Is Tirzepatide Made Of? The Peptide, the Fatty Acid Chain, and What Goes Into Compounded Versions

Tirzepatide is made of a 39 amino acid peptide modeled on GIP, a gut hormone released after meals, with a C20 fatty acid chain attached so that the molecule binds to albumin in the blood and lasts about a week. In the branded injection, it is dissolved in water with sodium chloride and a phosphate buffer.

Compounded tirzepatide is prepared by pharmacies rather than manufactured under an approved application, so its ingredients, salt form and added vitamins can differ, and the FDA does not review it before it is sold.

What is tirzepatide made of?

Tirzepatide is made of a single chain of 39 amino acids joined to a fatty acid. Coskun and colleagues, the Eli Lilly team that first described the molecule (then called LY3298176) in Molecular Metabolism in 2018, report that it "is a 39 amino acid linear peptide conjugated to a C20 fatty diacid moiety via a linker connected to the lysine residue at position 20."

The Zepbound prescribing information adds that the peptide contains aminoisobutyric acid (Aib) in positions 2 and 13, ends in a C-terminal amide, and carries 1,20-eicosanedioic acid on the lysine at position 20. The label lists the molecular weight as 4813.53 Da and the empirical formula as C225H348N48O68.

The peptide backbone. The amino acid chain activates receptors. Built on the GIP sequence and engineered to also activate the GLP-1 receptor, tirzepatide works on both incretin receptors, which is why it is called a dual agonist.

The two Aib residues. Aminoisobutyric acid is not one of the amino acids the body uses to build its own proteins, and Coskun and colleagues describe the two Aib units as "non-coded amino acid residues".

The C-terminal amide. The end of the peptide chain is capped as an amide rather than a free acid.

Why does tirzepatide have a fatty acid attached?

The fatty acid chain lets tirzepatide ride on albumin, a major blood protein, which slows how quickly the drug is cleared. The Zepbound label states that tirzepatide "contains a C20 fatty diacid that enables albumin binding and prolongs the half-life," and reports that the drug is 99% bound to plasma albumin.

With the fatty acid attached, the label gives an elimination half life of approximately 5 to 6 days in people with overweight or obesity. In the first human studies, Coskun and colleagues measured a mean half life of approximately 5 days (116.7 hours), which they wrote supported a once weekly schedule. The label notes that steady state levels are reached after about 4 weeks of weekly injections.

After an injection under the skin, the label reports a median time to peak concentration of 24 hours (range 8 to 72 hours) and a mean absolute bioavailability of 80%. Exposure increases in proportion to dose, a pattern visible in the phase 1 data.

How is tirzepatide made?

Tirzepatide is made by chemical synthesis, assembling the amino acids in sequence and attaching the fatty acid side chain. The discovery paper states that the peptides studied "were synthesized at Eli Lilly and Company using traditional peptide chemistry methods."

At scale, the main method is solid phase peptide synthesis, in which the growing chain is anchored to resin beads while each amino acid is added. Pang and colleagues at Sichuan University, writing in Organic Letters in 2025, note that the synthesis of tirzepatide "relies heavily on solid phase peptide synthesis (SPPS), a process that is both costly and time-consuming," and describe a liquid phase alternative intended to reduce wasted amino acids and solvents.

For an approved product, the manufacturing process and its quality controls are part of what the FDA reviews.

What are the inactive ingredients in Zepbound?

The inactive ingredients in Zepbound are sodium chloride, sodium phosphate dibasic heptahydrate and water for injection, plus three more in the multidose presentations. According to the label, each single dose pen or single dose vial holds 0.5 mL of solution containing tirzepatide, sodium chloride (4.1 mg), sodium phosphate dibasic heptahydrate (0.7 mg) and water for injection.

The multidose vial and the KwikPen each hold 2.4 mL, enough for 4 doses of 0.6 mL. In these presentations, each dose also contains benzyl alcohol (5.4 mg), glycerin (4.8 mg) and phenol (1.08 mg), along with sodium chloride (1.05 mg) and sodium phosphate dibasic heptahydrate (0.8 mg). Hydrochloric acid or sodium hydroxide may be added to adjust the acidity, and the finished solution has a pH of 6.5 to 7.5.

Single dose strengths of 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg and 15 mg share this ingredient list, and only the amount of tirzepatide changes.

What is compounded tirzepatide made of?

Compounded tirzepatide is made of tirzepatide active ingredient that a pharmacy buys from a supplier and prepares as a solution or another dosage form, and its exact composition depends on the pharmacy. The FDA states that "compounded drugs are not FDA approved," which means the agency "does not review compounded drugs for safety, effectiveness or quality before they are marketed."

Added ingredients. Belcourt and colleagues at the University of Connecticut searched compounding websites in early 2025 and identified 33 compounded semaglutide or tirzepatide products that differed from the approved versions. Of those, 48% combined the drug with some mix of cyanocobalamin (vitamin B12), glycine, niacinamide, docusate or ondansetron. The authors, writing in the Annals of Pharmacotherapy in 2026, found "little justification for adding nutrients or docusate sodium."

Different dosage forms. The 17 single ingredient products in that review were all sublingual or orally disintegrating tablets. Zepbound is labeled only as an injection for subcutaneous use, and the authors describe the efficacy and safety of these compounded formulations as largely unknown.

Salt forms and ingredient sources. When it launched a "green list" import alert for GLP-1 active ingredients in September 2025, the FDA said it had previously identified serious concerns with compounded tirzepatide, including dosing errors, the use of unapproved salt forms and adverse events, some requiring hospitalization.

A separate analysis by Eli Lilly scientists, published in Expert Opinion on Drug Safety in 2026, tested compounded tirzepatide products combined with B12 and reported "a widespread and previously unidentified impurity" formed by a reaction between tirzepatide and certain B12 analogs. The analysis came from the maker of the branded drug, and the authors state that the clinical effects of the impurity are unknown.

How is compounded tirzepatide made, and who makes it?

Compounded tirzepatide is made either by pharmacies working under section 503A of federal law or by outsourcing facilities under section 503B, and the two follow different rules. According to the FDA, 503A compounding must be based on valid patient specific prescriptions, while 503B outsourcing facilities "are subject to CGMP requirements" and are inspected by the FDA on a risk based schedule.

The rules changed when the national shortage ended. Without a shortage, compounders generally may not make products that are essentially copies of an approved drug, which helps explain why many compounded products now include additives or use different forms.

Is compounded tirzepatide the same as Zepbound or Mounjaro?

Compounded tirzepatide is not the same product as Zepbound or Mounjaro, because no regulator has confirmed that its identity, strength and purity match the approved drug. The FDA reports that as of May 31, 2026 it had received more than 730 reports of adverse events associated with compounded tirzepatide.

Reporting data add context without settling cause. McCall and colleagues reviewed 81,078 GLP-1 reports in the FDA Adverse Event Reporting System from 2018 to 2024, of which 707 involved compounded products. Figure 5 shows where the reporting odds differed.

The authors note the study was not designed to show cause and effect. People prescribed a compounded product can reasonably ask whether the pharmacy operates under 503A or 503B, whether the ingredient is a salt form, which additives are included and why, and how the product is tested. A licensed clinician can explain how a specific formulation compares with an approved option, and our comparison of tirzepatide and semaglutide covers how the active drugs differ. Individual results vary.

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