Beware of Teriparatide mixed with other ingredients! Eli Lilly(LLY.US) warns that combining cheap weight-loss drugs carries hidden risks.

Eli Lilly and Company (LLY.US), a leading figure in the U.S. healthcare industry, has warned that the core active ingredient in its blockbuster weight-loss drug Zepbound—after being mixed with vitamin B12—could pose a major risk to patients’ health, and that this formulation is being sold by U.S. prescription compounding pharmacies.

On Thursday, Lilly said in a statement that the company tested the compounded product and found it contained “significant levels of a certain impurity,” with the impurity arising from a chemical reaction between vitamins and the active ingredient in Lilly’s weight-loss drug tirzepatide.

The U.S. pharmaceutical company said: “Anyone who obtains a tirzepatide–B12 product from a compounding pharmacy, a telemedicine company, a medical aesthetics institution, or any other channel should understand that the product they may be using is one that carries potential danger and has unknown risks to the human body.” The company also mentioned the possibility of excessive immune reactions that could be toxic and harmful to health.

Like Novo Nordisk, the strongest competitor in the weight-loss drug market, Lilly is also facing pressure from cheaper generic and compounded versions. These replicated injection drugs for weight loss and diabetes are circumventing the stringent regulatory review and approval processes required for branded drugs or generic drugs.

The reason these generic and compounded drugs were allowed for a period to be sold in the U.S. market was to address extreme supply tightness that emerged in the early days of the weight-loss injection frenzy. Soon after, the U.S. Food and Drug Administration (FDA) announced that the shortage had ended, which also means these discounted compounded and generic versions are no longer allowed to be sold officially.

Even so, some generic and compounding pharmacies continue to take advantage of a regulatory loophole to sell their products. The loophole allows them to make exclusive customized changes to drug formulations—for example, slightly adjusting dosages, or adding other ingredients such as vitamin B12.

Lilly, it is understood, has already been urging the U.S. Food and Drug Administration to recall all compounded tirzepatide products that are mixed with additives not tested against official standards.

Lilly said that this latest round of testing also found other key safety issues, such as bacterial contamination and endotoxins—high levels of toxic ingredients.

Lilly said these compounded weight-loss drug versions are mixing tirzepatide with chemical substances such as glycine, pyridoxine, niacinamide, and carnitine, creating a range of new untested combination drugs. The company urged the U.S. Food and Drug Administration to recall all combination drugs containing untested additives.

Earlier, the FDA had issued warnings to more than 30 telemedicine companies regarding misleading promotion of compounded GLP-1. The regulator’s enforcement direction appears to be increasingly aligned with that of branded drug manufacturers.

From the perspective of sales and expectations for earnings growth, Lilly’s safety warning about its “tirzepatide + B12” compounded weight-loss drug is overall more favorable to its branded weight-loss drug sales outlook—particularly in terms of Zepbound’s share recovery and restoration of pricing power in the U.S. cash-pay market. For Lilly, if the FDA recalls these untested compounded products, that would mean demand previously diverted to lower-priced compounded tirzepatide would more likely flow back to compliant branded channels afterward. However, because the U.S. weight-loss drug market itself is also undergoing price cuts and cash-pay competition, it doesn’t necessarily translate into an immediate surge in newly added demand.