http://www.felinecrf.org/vitamin_b.htm#methylcobalamin_injectable
Vitamin B12 Injectable
Injectable vitamin B12 is only available via prescription in the USA, but is an over the counter product in the UK and Canada. See below for stockists, including a US supplier of injectable methylcobalamin.
Unlike vitamin B complex, injectable vitamin B12 (which is a pinky red colour) does not sting, so some people give this to their cats during or immediately after sub-Qs.
The US National Library of Medicine mentions that many injectable forms of vitamin B12 in the form of cyanocobalamin "contain aluminum that may be toxic. Aluminum may reach toxic levels with prolonged parenteral administration if kidney function is impaired." If you are only using the injectable form occasionally, it is probably safe but discuss this further with your vet. The Anivit injectable B12 available in the UK is aluminium-free. Injectable methylcobalamin apparently does not contain aluminium (the version I know of is preserved with methyl alcohol).
If you do use an injectable form of vitamin B12, the
Centers for Disease Control and Prevention recommend discarding multi-dose vials within 28 days of starting them.
Vitamin B12 Injectable Dosages
Doses are lower than for oral administration because less is wasted when the injectable form is used.
A commonly used dose for injectable cyanocobalamin is 250-500mcg every week or every two weeks.
Drugs recommends 0.25 to 0.5 ml of the 1000mcg/1ml concentration (so 250-500mcg), every 1-2 weeks as required
Although it is more bioavailable than cyanocobalamin, I understand from those who have used injectable methylcobalamin that the dosage is the same, i.e. 0.25ml of the 1000mcg/ml strength (which is 250mcg), but it is normally only given every 2-4 weeks, although some members of Tanya's Support Group give it more often. Be guided by your vet on the best protocol for your cat.