Sounds like an accurate diagnosis based on the symptoms. I would just use a normal b-complex vitamin for humans. I get a capsule form so I can open the capsule to easily dose. This is perfectly safe to give to chickens.
Since vitamin b is water soluble, overdosing isn’t as much of a danger as with some other vitamins but should still be avoided as a best practice. Niacin (b3) specifically can be toxic in extremely large doses.
If vitamin b support is needed, I would dose a chicken with about 1/20th of the human dose daily. I guess my method is a bit over complicated but here it goes. I open up the capsule and measure out about 1/10th of the powder. I store the rest in a small, airtight nalgene container away from any sunlight. I thoroughly mix the 1/10th with some water (probably 2ml) and then I dose the chicken with about 1/2 of the mixture. I just use a small syringe and drop it into their beak from the side and wait and allow the chicken to swallow it. I try to use the vitamin b/water mixture right away as the riboflavin (b2) can degrade quickly in direct sunlight.
I don’t use the rooster booster liquid b12 as it is pricey and it only contains b12. Chickens do love it because it’s basically sugar syrup.
My preferred b complex for myself is b right from Jarrow. Perhaps a bit fancy for poultry but I always have it handy around the house. https://www.amazon.com/Jarrow-Formulas-B-right-Supports-Cardiovascular/dp/B0016003Z0
It would be more economical to use the b complex from Costco but so far I’ve only found the Kirkland brand in a pill form and not a capsule. I expect you could pick up any generic brand of vitamin b complex in a capsule from your local supermarket and use it.
If you have sterile, veterinary-grade syringes & needles and are comfortable giving IM injections then that is my preferred method. Durvet makes a good injectable vitamin b complex you can pick up from your local farm store for about $10 usually. Good luck!
Jarrow Formulas B-right Complex, Supports Engery, Brain and Cardiovascular Health, 100 Veggie Caps https://www.amazon.com/dp/B0016003Z0
you're welcome. just make sure you take a multivitamin as well, because you need stuff like magnesium , vitamin d, vitamin k, b vitamins and I will stop there because I take like 35 supplements a day lol