How long can it take for B12 injections to take effect?

Hi, all!  I have a kitty who just turned 15.  She was diagnosed with hyperthyroidism in 2012, was on Felimazole for four years, and finally got the 131 treatment this summer.  She had to be dosed twice, as her condition was quite entrenched.  Now for the three months since her first treatment, she has not been eating well at all and has lost two pounds.  She is euthyroid, her Chem 10 panel came back good for kidney and liver function, but despite appetite stimulants and antacids/antiemetics, she only wants to fill her belly with water constantly. 

Yesterday I brought her to the vet for her first ever B12 injection.  Maybe I was expecting too much; she is more active now, happier, but is not any more interested in food than she was before.  How long does it take, on average, for the appetite stimulant effect to kick in?  Kinda worried sick about my girl.  It's like she wants to waste away.

Hello @HWolters, I have a similar situation with a 19 year old kitty. He has been on 5mg felimazole for years, otherwise doing well. Though his appetite is good, I give him a B-12 shot every month. It helps his alertness and activity level. When the B-12 treatment began, I had to give my boy a shot once a week for a month, then one every month.  

You might try some appetite stimulant, or some turkey or chicken baby food. Also, check to see if you need to start at B-12 regimen, it may help in the long run. 

Keep us posted!