B12 vs L-Carnitine in Compounded Semaglutide and Tirzepatide: What the Additives Do, What the Evidence Says, and Why They Are There

The question behind the question

When patients ask what the medication is compounded with, they are asking two things: is there anything in this vial I should worry about, and why is it there. Both deserve straight answers, and the second is more interesting than the first.

What is in an approved product

Wegovy's inactive ingredients are disodium phosphate dihydrate, propylene glycol, phenol and water for injection. Zepbound's are sodium chloride, sodium phosphate dibasic heptahydrate, water, and hydrochloric acid or sodium hydroxide for pH. That is all. The manufactured products are the peptide in a buffered, preserved aqueous solution.

What is in a compounded product

A compounded semaglutide or tirzepatide vial contains the peptide (it should be the base peptide, not a sodium or acetate salt — the FDA has warned about the latter), water, a preservative to allow multiple withdrawals from one vial (benzyl alcohol or phenol), a buffer, and, at many pharmacies, one of two additives: vitamin B12, as cyanocobalamin or methylcobalamin, typically in the hundreds of micrograms per mL, or L-carnitine, typically in the tens of milligrams per mL. Some pharmacies also offer combinations with pyridoxine (B6), NAD+ or glycine, but B12 and L-carnitine are by far the most common.

The formulation varies by pharmacy, not by telehealth brand, and a provider that uses several pharmacies may ship different formulations to different states.

Vitamin B12: the claims and the evidence

The marketing claims for B12 in a GLP-1 are that it boosts energy, counteracts fatigue during weight loss, and reduces nausea. The evidence is as follows. B12 deficiency causes fatigue and neurological symptoms, and correcting a deficiency resolves them; supplementing B12 in a person who is not deficient has not been shown to improve energy in randomized trials. B12 has no established antiemetic effect; the antiemetics with evidence for GLP-1 nausea are ondansetron and, for some patients, dietary adjustment and slower titration. No randomized trial has compared a GLP-1 with and without B12 on weight loss, nausea or energy.

There is one legitimate clinical rationale. Metformin, which many patients with type 2 diabetes or prediabetes take alongside a GLP-1, reduces B12 absorption, and B12 deficiency is common in long-term metformin users; the American Diabetes Association recommends periodic B12 testing for them. For such a patient, B12 in the vial is reasonable, though a separate oral or injected supplement at a known dose is more controllable. B12 is water-soluble, excess is excreted, and the doses used are safe. Cyanocobalamin and methylcobalamin are both effective at raising B12 levels; the marketing preference for methylcobalamin as "more bioavailable" is not well supported at these doses.

L-carnitine: the claims and the evidence

L-carnitine transports fatty acids into mitochondria for oxidation, which is the basis of the claim that it "burns fat." The body makes its own carnitine and most people are replete. A 2016 meta-analysis of L-carnitine supplementation for weight loss found a small mean difference (about 1.3 kg over trials of varying length) with oral doses of 1 to 4 grams per day — far above the tens of milligrams in a weekly compounded injection — and no trial has tested L-carnitine added to a GLP-1. Whatever the oral-supplement literature shows, it does not transfer to a 25 mg weekly injected dose.

There is a safety question that is at least worth knowing about. Oral L-carnitine is metabolized by gut bacteria to trimethylamine, which the liver converts to trimethylamine N-oxide (TMAO), a compound associated with atherosclerosis in mechanistic and observational studies. Injected carnitine bypasses the gut and the doses are small, so the relevance is unclear, and no adverse signal has been reported from compounded GLP-1 formulations containing it. But "no evidence of benefit, theoretical question about harm" is a weaker profile than B12's "no evidence of benefit, clear safety."

Why the additives are there

The candid answer has two parts. The first is marketing: "semaglutide with B12 for energy" is a product feature. The second is regulatory, and it matters more since 2025. When the FDA removed tirzepatide (December 2024) and semaglutide (February 2025) from the shortage list, compounding "essentially a copy" of the approved products became impermissible. What remains permitted is compounding for an individual patient with a documented clinical need for a formulation that differs from the approved product. An additive makes the formulation differ. The FDA has said it will not regard a change made solely to evade the copy prohibition as a legitimate clinical need, and that a prescriber must document the individual patient's need; whether "the patient needs B12" satisfies that standard is a question regulators and courts are still working through, and no patient can resolve it from a website.

What a patient can do is understand that the additive's presence is not evidence that it helps, and ask the prescriber what the documented need is.

What to ask the pharmacy

• What is the complete formulation, including the additive and its amount per mL?
• Is the peptide the base form (semaglutide, tirzepatide), not a salt?
• What is the concentration of the GLP-1 in mg/mL? The additive does not change the dose arithmetic, but a new formulation often means a new concentration — recalculate with the dose calculator.
• What preservative is used? Benzyl alcohol is a common allergen and is avoided in some populations.
• Can I have the same medication without the additive? Some pharmacies offer both.

All of this should be on the label. A label that lists only "semaglutide" with no formulation details is a reason to ask more questions.

The bottom line

B12 in a compounded GLP-1 is harmless and, for a metformin user with low B12, mildly useful; it will not make you lose more weight or feel less nauseated. L-carnitine has no evidence of benefit at the doses used and a theoretical question about long-term oral use that probably does not apply to injections. Neither should influence your choice of provider, and neither should cost extra. What should influence your choice is whether the pharmacy tells you exactly what is in the vial. Our pharmacy guide and 503A vs 503B article explain how to check the rest.

Sources

1. U.S. FDA, FDA clarifies policies for compounders (Dec 2024, Feb 2025), on formulations that differ from approved products
2. Office of Dietary Supplements, NIH, Vitamin B12 Fact Sheet for Health Professionals
3. Office of Dietary Supplements, NIH, Carnitine Fact Sheet for Health Professionals
4. Koeth RA et al. Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Nat Med 2013;19:576-585
5. Pooyandjoo M et al. The effect of (L-)carnitine on weight loss in adults: a systematic review and meta-analysis. Obes Rev 2016;17:970-976
6. Wegovy and Zepbound prescribing information, inactive ingredients

Citations are to primary sources (peer-reviewed trials, FDA labeling and announcements, and provider pricing pages). See our sources policy and corrections log.