Intramuscular vs subcutaneous administration

Earlier today I posted a guess about IM vs SC administration of B12. I decided to so some looking around and found this paper -

The results showed, for many of the drugs studied, there was little difference between the two routes.

Some of the individual results were interesting. For epinephrine (given for anaphylaxis - where speed is important) peak concentrations were reached faster with IM. But when they looked at several different sites for IM administration of epinephrine they found a huge difference between injections into the thigh and arm (the thigh was much, much better - presumed to be because of greater blood flow).

For gonadotrophin and hydrocortisone they found that body size can affect the maximum blood concentration for SC doses - a higher BMI meant lower blood levels.

Then I found this document - - which agrees with my reasoning but doesn't give any data - just theory.

In the future I think I'll be suggesting that there's probably not much difference and, considering how SC is less scary than IM, SC may be preferable for many.

17 Replies

  • Thanks for posting the link to those documents. They look very interesting, though the 2nd is full of the kind of maths that used to blow my synapses (technical term) decades ago. I'm considering changing my dose in light of your comments.

  • So long as the product is not lost through excretion, it could be argued that the slower it is absorbed the better.

    I feel I ought to try SC but for now will stick to IM.

    Just in case anyone is interested....

    When dosing animals with vitamin and mineral supplements we usually use the IM route. We give calcium and magnesium supplements slowly IV as well as SC, as deficiencies of these present very acute symptoms with imminent death likely so fast uptake initially is essential. We usually give a second dose SC at the same time because that way it is absorbed slowly and provides a "reservoir" to boost levels during recovery. Phosphorus deficiency, which doesn't seem to be a major problem in people, is usually treated with a two part injection - half IM and half SC.

  • Interesting comments Deniseinmilden. Once again, I'm left wondering if a vet (or a Farmer) may be of more use than a GP. Ha ha!

  • Mine is... lots more so! I always ask my vet in preference to my GP!

  • Looks like our thinking was going in the same direction on this, deniseinmilden! :-)

  • Very interesting Fbirder. Gives much food for thought...and plenty of maths to show me how rusty I am!

    It's really good to get good solid research information so thank you for posting.

    Be really interesting to see if this offers better ways to manage B12 influx.

    Cheers 🍷

  • Thanks fbirder, for posting those thought provoking articles.

    I'm wondering about a mixed approach with SC & IM as discussed in the first article. That is, one day doing SC because of the slower release of the B12, while the next dose would be IM for faster delivery to the system, continuing with thus alternating pattern. If a person is using B12 several times a week, then over time, this 2 pronged approach might be helpful to keep a more steady dose in the system. Faster but more fleeting effects with IM combined with slower but longer acting with SC.

    I'd love to hear your thoughts on this, fbirder, and everyone else's, too! :-)

  • That may be really good news. Not only is SC less scary than IM, but long term IM injection does eventually cause muscle scarring. Plus most medical professionals are more willing to let patients inject themselves subcutaneously than intramuscularly. And, in the US, many states don't require a prescription for obtaining insulin-style syringes.

  • For me SC would also be better as I could do it myself and my husband wouldn't have to give me IM injections. Not that he minds, but he is a furniture maker and is working on a big project so I hate to interrupt him to give me shots.

  • P.S. I'm not as brave as all of you who SI yourselves! :(

  • The big question now is - why do the NHS use IM?

    My guess is that it's easier from the nurse's point of view as they do a lot more IM than they do SC.

  • Why does that not surprise me.

    If IM is so much safer, why don't diabetics use it?

  • If doing SC instead of IM - what size needle do you use?

  • I don't know what people do use, but I would go for an orange (25G) and probably 25mm.

  • I use 30g (really, really thin) x 1/2" for Hydroxy, the needle is so skinny, it really is a breeze to use, I am quite new to SI, 17 injections so far and couldn't believe how easy it was, all that apprehension for nothing LOL.

  • ... and this is hydroxy rather than methyl that is ok to use S/C ?

  • I wonder if IM is used because the focus has been on the need to restore the blood to a normal state as quickly as possible.

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