Because that is what the market sells. A secret-shopper study of 75 US clinics found 56% of them offering the compounded GLP-1 mixed with B vitamins [1]. No deficiency is measured before the B12 goes in, so it functions as a product differentiator. The same survey found 19% of the supplying facilities not licensed for sterile compounding at all.
A single-ingredient compound avoids the question entirely. The price gap between the two formulations is usually small. Run that comparison before the marketing copy. what a GLP-1 actually costs sets out the arithmetic.
What testing found in the combination
A group tested compounded tirzepatide-plus-B12 samples bought from various US sources, and found an impurity nobody had described before. It was present at substantial levels and widespread across the samples tested [2].
The tested products were tirzepatide combinations. The semaglutide equivalents sit in the same market and have not been through the same analysis. For them the question is open rather than answered. The market itself is described in what the secret-shopper study found.
What the paper does not say
The clinical effect of the impurity is unknown, and nobody has tested what it does to a person. No patient harm is reported or alleged anywhere in the work.
Nothing here is a reason to stop a course mid-way. It is a finding about what is in the vial, not a measurement of harm. That distinction separates it from the dosing errors a poison control center actually logged.
Funding is the other open question. Papers testing compounded copies of a branded drug are sometimes written by people employed by the original’s manufacturer. That was the case in the semaglutide impurity work. This paper’s abstract does not state its funding. The finding stands or falls on whether another laboratory reproduces it.
Who is watching the combination
Fewer people than a buyer would assume. A survey of clinicians and pharmacists found 51% aware of the 2024 guidance on compounded GLP-1 products [3]. Most of them reported low confidence and no direct experience with the products.
So a prescriber asked about a B12 combination may have no more information than the buyer does. That is a gap in the chain rather than a criticism of anyone in it. It is set out in half the clinicians had not heard of the guidance.
The rule this desk applies
A combination product carries whatever the combination does. Where a plain single-ingredient compound is available at a comparable price, the added ingredient has to earn its place on evidence rather than on the label.
Marketing is not evidence of a measured deficiency, and nothing about a compounded formulation is reviewed before it is dispensed. That is the underlying condition described in what the label need not tell you.