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Thanks for the question.While I cannot diagnose this over the internet I can tell you the following:1. Vitamin B12 is a drug which all small animal vets will be familiar with and in my experience it has a very high margin of safety. While side effects are always possible with any drug the chances of this happening with B12 would be very small indeed.2. Since this drug is given as a treatment for a disease condition in your case I would say that the balance of probability would be that the symptoms you are seeing are related to a resurgence of the infection and not the B12.3. The safest thing you can do here is to consult your vet again as soon as is practicable for you ...If I have not covered your question fully enough or you would like to ask more I will be online for the next hour or so and I will be at your disposal.Scott Nimmo BVMS MRCVS