Pharmaceutical company Eli Lilly and Company has issued an open letter warning healthcare professionals and patients about potential safety risks associated with compounded versions of the drug tirzepatide when mixed with vitamin B12.
The warning follows laboratory testing conducted by the company on compounded products marketed in the United States that combine tirzepatide with vitamin B12. According to the company, the tests detected significant levels of an impurity created by a chemical reaction between the two substances.
Tirzepatide is the active ingredient in Lilly’s FDA-approved medicines Mounjaro (for type 2 diabetes) and Zepbound (for chronic weight management).
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In the open letter, Lilly reported that its testing identified impurities in all samples of compounded tirzepatide products that also contained vitamin B12 as a result from a chemical interaction between tirzepatide and vitamin B12. The long-term or short-term health effects of this impurity remain unknown.
According to the company, such chemical changes could potentially affect how the drug behaves in the body, including how it interacts with its biological targets, the GLP-1 and GIP receptors, which are involved in glucose regulation and appetite control.
Lilly noted that tirzepatide has not been clinically studied in combination with vitamin B12, and therefore the safety and effectiveness of such formulations have not been established.
Tirzepatide is a dual incretin receptor agonist that targets both glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptors.
These medications are prescribed to improve glycemic control in adults with type 2 diabetes and to support chronic weight management in adults with obesity or weight-related medical conditions.
The FDA-approved versions of tirzepatide are administered as subcutaneous injections and are available only with a prescription under medical supervision.
Compounded medications are customized drugs prepared by pharmacists to meet specific patient needs, such as adjusting dosage or removing certain ingredients. However, unlike FDA-approved medicines, compounded drugs are not evaluated by the U.S. Food and Drug Administration for safety, quality, or efficacy before being marketed.
Demand for GLP-1–based medications used for diabetes and weight management has increased globally in recent years. During periods of supply shortages, some compounding pharmacies began producing alternative formulations of drugs containing the same active ingredients.
Some of these compounded products have included additional substances such as vitamin B12, glycine, or other vitamins, which are sometimes marketed as personalized formulations. However, the safety and clinical effects of these combinations have not been studied in large clinical trials.
The company emphasized that medications labeled simply as “tirzepatide” but not marketed under approved brand names may not have been manufactured, studied, or approved by regulatory authorities.
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