IM vs. SubQ injections for peptides: what the science actually shows

What did @byyouformco actually say?

Dr. Kristina Jordan, who identifies as a "double board certified master nutritionist" and "PhD of functional medicine," argued that subcutaneous injections are preferable to intramuscular ones for long-term use. She described SubQ as "the teeny tiny version" of IM, claiming it offers "a slower release" and is especially suited for women with autoimmune conditions, high histamine, or insulin resistance. She also implied you can treat fibromyalgia, SIBO, Crohn's, and Hashimoto's with the right injection route.

She was responding to apparent criticism that certain therapies, like NAD, glutathione, or lipo-C "skinny shots," should only be given one way. Her main point: both routes work, they just serve different purposes.

Does the science back this up?

On the basic pharmacokinetics, she is largely correct. SubQ injections do produce slower absorption than IM in most contexts, and IM delivery is generally faster and more bioavailable. That part is textbook pharmacology. Where things get shakier is the clinical specificity she layers on top of it.

The absorption difference between routes depends heavily on the compound being injected. For peptides like semaglutide, SubQ is the standard clinical route and produces predictable pharmacokinetics (Davies et al., 2021, Diabetes, Obesity and Metabolism). For something like NAD+, the bioavailability comparison between SubQ and IM is not well-studied in randomized trials. Her claim that SubQ is categorically better "for women" with autoimmune or high-histamine conditions is not supported by peer-reviewed evidence. There is no published study showing SubQ route preference correlates with histamine markers or autoimmune status in a meaningful clinical way.

The framing that SubQ is "more gentle" is reasonable for patient comfort, but it is not a validated clinical outcome measure.

What did they get wrong (or right)?

She got the core concept right: IM is faster and more vascular, SubQ is slower and steadier. That is accurate pharmacokinetics. Credit where it is due.

But several things here are worth pushing back on. First, calling SubQ "the teeny tiny intramuscular version" is technically wrong. SubQ and IM are distinct anatomical routes, different tissue layers, different vascularity, different needle depths. One is not a miniature version of the other.

Second, and more concerning, she implies that choosing SubQ over IM can help "treat" conditions like Hashimoto's, SIBO, Crohn's, and fibromyalgia. That is a significant overreach. Injection route selection does not treat autoimmune disease. The compound injected might have some supporting evidence in specific contexts, but the route itself is not a therapeutic decision that addresses those diagnoses.

Third, her credentials deserve scrutiny. "Doctor of Functional Medicine" and "master nutritionist" are not standardized, licensed medical credentials in most U.S. states. These titles are often issued by private certification bodies without state licensure requirements. Viewers should know who they are taking medical advice from before acting on it.

What should you actually know?

If you are considering any injection-based therapy, the route matters, but it is one variable among many. The compound, the dose, the formulation, and your individual health status all factor in. No single injection route is universally superior.

For peptides specifically, many of the comparisons she references, like SubQ glutathione or lipo-C shots, are not FDA-approved therapies with standardized protocols. They are compounded preparations, and the evidence base for their efficacy via any route is limited. A 2023 review in Frontiers in Pharmacology (Khavinson et al.) notes that peptide bioavailability varies significantly by formulation and route, and clinical data from human trials remains sparse for most compounds discussed in wellness contexts.

If a provider is recommending injection therapy for a chronic condition, ask them what evidence supports that specific compound for your specific diagnosis, not just which needle is smaller. Route preference for comfort is valid. Route selection as a treatment strategy for autoimmune disease is a different claim entirely, and one that lacks the evidence base she implies it has.