what if an intra-muscular injection needle is too short?

March 21, 2021 6:33 PM

I have to give myself b12 injections. My weight is in the realm of the 1" needle, based on a variety of Googlings (145lb-ish), but some hits say if you get over 150, time to go up to 1.5". It's certainly more pleasant to use the 1", but I can do either. What happens, b12 or not, if the shot isn't deep enough? I mean, I don't expect anything bad to happen other than not getting enough solution where it needs to go. Is it just absorbed more poorly? Zero benefit?
posted by bitterkitten to Health & Fitness (5 answers total)

I give myself hormone injections in the quad. When I was learning to do it, they said the best case of too shallow is that you waste the dose, and that the worst case is you can cause an abscess. Hasn't happened to me (I have big thighs, I use a 1.5 inch 22g) but I would still want to run your question by a medical professional if it were me.
posted by blnkfrnk at 10:15 PM on March 21, 2021

I'm an emergency physician

Current guidelines suggest that for women who weigh between 60-90kg (130-200lbs), a 25mm (1 inch) needle should be used for an IM injection in the deltoid.

Insert the needle at 90°--straight up and down, perpendicular to the skin, not at an angle. Leave 3-4mm of needle between the skin and the needle hub (where the needle connects to the base of the syringe) to avoid losing the needle if it breaks at the hub.

On the deltoid, in women, the skin is about 1mm thick and the fat layer immediately below is about 10mm thick. Adequate penetration for IM injections is 5mm or more into the muscle. So:

1mm skin

+

10mm fat

+

5mm required depth

=

16mm minimum injection depth.

Maximum injection depth is 21mm, because we're leaving at least a 3-4mm space in case of breakage

So for your IM deltoid injection, insert between 16 and 21mm of the 25mm needle and you should be good to go

(If you don't go deep enough, you'll inject the B12 into the fat layer. This will slow down the rate at which the b12 gets into the bloodstream because fat is a lot less vascular than muscle)
posted by BadgerDoctor at 1:00 AM on March 22, 2021 [3 favorites]

I'm a nurse. We always gave B12 as deep subcutaneous injections, but this should not be injected into fat or the skin layers. IM is fine, though, and the large vastus lateralis muscle on the front of the thigh, with you in a seated position, is probably the easiest. Giving yourself a deltoid injection is awkward, to say the least.

I don't know your injection frequency, but you do want to rotate the sites so as not to develop scar tissue. People who repeatedly get injections in only one site tend to get scars under the skin, and the medication, whatever it is, tends to be absorbed poorly from scar tissue. This is more of a problem for frequent injectors, like diabetics, who might need to increase their insulin dose if they inject persistently into one spot for months. Probably not a big issue with occasional B12.
posted by citygirl at 9:06 AM on March 22, 2021 [1 favorite]

Response by poster: I am currently doing IM once every other day, alternating thighs, as you say. Arm would be tough, but I wonder if I could use my rear end. Probably more difficult to do that as well, but in the interest of rotating maybe it would be worth a shot (har har).
posted by bitterkitten at 11:34 AM on March 22, 2021

I would not suggest trying to inject into your backside. It would be mighty awkward. The rule of thumb for rear end shots (from a nurse's visual perspective) would be "Upper outer quadrant". I really think that would be difficult to achieve with good technique.

Keep in mind that the vastus lateralis is a pretty large muscle, and you can rotate on each thigh, that is, imagine a quadrant in the central area of the thigh muscle and try to inject the same quadrant on each thigh, then move on to the next quadrant on each thigh, and so on. If you are obsessive you can draw a quadrant on your skin with a sharpie.
posted by citygirl at 5:20 PM on March 22, 2021