Marti thought night was day when her vitamin B12 nosedived, also was depressed and lethargic.
. It is the same with humans. The vet, like my GP , will poo-poo you, and the specific test is expensive. However, B12 is harmless and any excess is excreted in the wee. My vet , ( not mine any more!)reluctantly gave her an injection, and overnight she became a new dog. If you want to avoid vets, you can get B12 capsules for dogs from Chemeyes.co.uk and give one a day for a week, then one a week.
Much has been written recently on humans diagnosed with dementia actually suffering from B12 deficiency, and perking up immediately . My GP wanted to send me to a Memory clinic, with my B12 level rock bottom. I took the vitamins instead and my brain fog and blood levels recovered quickly.