Firstly, I am no expert. I am just someone who had B12 deficiency, and eventually, reluctantly, had to self inject in order to effectively control my B12 deficiency symptoms.
I have now been self injecting since 2017 and I am still increasingly able to control my symptoms. This is not the same thing as a cure, and not all symptoms for all people are reversible .
For those who have not had the benefit of being taught how to do this by a GP or nurse, there are an astonishing amount of videos online. Conflicting advice and fear and anxiety (which can be "normal" or a B12 deficiency symptom or both) really doesn't help. What if you choose the wrong person, what if they don't know what they are talking about ? Without doubt, it is a scarey thing to do.
I was inept at first - no, really rubbish at it, being honest.
These are the things that I learned:
1) Clean:
Make sure the area you are using is clean. Make sure you don't expose equipment to air until ready.
2) Relax:
This is virtually impossible the first few times -and I think the main reason it goes pear-shaped initially. Give yourself plenty of time. You may need to tense muscles in order to ensure you know where you are heading, but then relax muscles fully so it doesn't feel like a fight at all. Have all your kit ready.
3) Stinging:
Stinging can be from using pre-injection wipes (not everyone does) and then not waiting for the skin in injection site to dry out.
Stinging can also occur if there is a droplet of B12 remaining at the tip of the needle when injecting, so flick the syringe to remove it first.
4) Aching:
Aching when injecting can be caused by B12 being kept in the fridge, then used without warming up sufficiently: the liquid thickens when cold. Some have advised putting the ampoule in their bra just while getting their equipment ready. Not useful for all of us, but a nice image.
Aching can also be due to injecting too quickly: pressing the plunger of the syringe sloooowly and smoothly will prevent this.
5) Bleeding:
The only time I bled significantly was early on: on removing the needle, blood immediately spurted in a huge arc across the duvet ! It lasted just long enough for me to panic - a few seconds that felt like a lifetime - then stopped. I must have hit a small blood vessel. Unfortunately, at the time, I was demonstrating to my partner how simple self injection is !
Yes, I get the occasional dribble of blood run down my leg or a small bruise later. But rarely. I think waiting a second to remove needle smoothly, and slight pressure after on injection site might prevent this.
You can also cut yourself when breaking the ampoule. I never have, even when I smashed one to smithereens. This is because I followed youtube advice from a man with sensible pants (precisely why I chose him above the multitude of others):
"use the syringe wrapper; wrap this around the ampoule top and then snap it back, away from you, with the dot on the neck of the glass ampoule facing you."
6) Sensitivity:
I think that my skin becomes more sensitive after a bath, so I inject before not after.
I very recently read on here that if you stretch your injection-site skin out with thumb and forefinger while you are injecting, it lessens likelihood of pain. It works for me, but may be a step too far initially. Too many parts to the process, and not enough hands !
7) Insensitivity:
I try to alternate between left and right legs. It isn't easy to lean across and do "the other one" but if you keep using exactly the same area on the same leg, and have to self inject frequently (not everyone does) or for a very long time, skin might start to get more difficult to get through with the needle. Finer needles may be more useful, but there is an increased risk of needle breaking allegedly - I have never "gone thinner", and do not know the statistics on this.
8) Time:
In time, it does get easier. At first, I think most of us rush to get it over with. You learn the best time of day for you and where your best places are.
If it hurts at all, or you feel like you are hitting stone, just stop.
Pull out the needle, breathe, relax, try again somewhere else.
It shouldn't hurt.
Don't forget the treat afterwards - you've earned it !
PS: I'm not medically trained and this is just what I have picked up by having to do this often, from sound advice from people here -and from the man with the sensible pants in the youtube video !
Have you tried getting stuff out of an ampoule with an 8mm needle?
Needle size for Surgery injections? 
Self-injecting question
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