Compounded Tirzepatide vs Mounjaro and Zepbound: Purity, Efficacy and Side Effects

Mounjaro and Zepbound are FDA-approved, which means the agency reviewed their safety, effectiveness and manufacturing quality before they went on sale.

What did testing find in compounded tirzepatide?

Many compounded tirzepatide products add vitamin B12 to the same vial. Eli Lilly, which makes Mounjaro and Zepbound, tested tirzepatide-and-B12 products sold in the US and found "significant levels" of an impurity, a new substance that forms when tirzepatide and B12 react with each other (Lilly's open letter). Lilly scientists published the results in the journal Expert Opinion on Drug Safety (the published testing).

No one knows what this impurity does in the body. According to Lilly, there is no information on its short- or long-term effects in people, whether it changes how tirzepatide acts on the appetite and blood-sugar receptors it targets, whether it is toxic, whether it triggers immune reactions, or how the body absorbs and clears it. Tirzepatide has never been studied in combination with B12. Unknown cuts both ways: no one has shown the impurity is harmless, and no one has shown it causes harm.

Keep two facts in mind when weighing this. Lilly competes with compounders and has called for a recall of tirzepatide/B12 products, while compounders argue their products are legal and safe (Reuters). And the testing covered B12 combinations only, so it tells you nothing either way about compounded tirzepatide without B12. If you use a compounded version, check the label for B12, which may be listed as methylcobalamin, hydroxocobalamin or cyanocobalamin.

Does compounded tirzepatide work as well as the brand?

Nobody has shown that it does through the review process the brand went through. Mounjaro (for type 2 diabetes) and Zepbound (for weight management) earned approval with evidence FDA checked. Compounded tirzepatide uses the same active ingredient but is made by compounding pharmacies without that approval, and no FDA-reviewed study compares its weight loss or dose accuracy with the brand. A compounded product may perform like the brand or it may not. The point is that no independent review has confirmed which.

The rules around compounded tirzepatide are also shifting. On April 30, 2026, FDA proposed removing tirzepatide from the list of raw ingredients that outsourcing facilities can compound from (FDA's proposal). Outsourcing facilities are larger compounding operations, sometimes called 503B facilities after the section of law that covers them. In most cases they can only compound from a raw ingredient if it appears on that list or if the drug is in shortage. FDA said it found no clinical need for these facilities to compound tirzepatide this way. FDA announced this as a proposal, not a final decision. Compounded tirzepatide was still widely sold in early 2026, and FDA has sent warning letters to some compounders over misleading claims (Reuters).

What side effects show up, and who tracks them?

For the brand, FDA keeps a database where patients, doctors and drugmakers can report side effects and other problems. An analysis of about 66,000 reports for Mounjaro and Zepbound from 2022 through early 2025 found that the most-reported problem was taking or being given the wrong dose (analysis of FDA side-effect reports). Dosing-error reports rose from 1,248 in 2022 to 9,800 in 2024. Injection-site pain and nausea were also among the most common reports. The authors said the rising number of reports points to a need for clearer dosing instructions and better education for patients and prescribers.

These are report counts, not rates. They show how many reports were filed, not how often each problem happens among people taking the drug.

That database analysis covers brand tirzepatide only. According to Lilly, compounders are not required to monitor or report side effects (Lilly's open letter), so there is no comparable count for compounded versions. The missing count doesn't show that compounded tirzepatide causes more problems or fewer. It means problems with a specific compounded product are harder to spot and trace back to it. You can report a side effect from any version, brand or compounded, to FDA yourself (FDA's reporting page).