Because the B vitamins are water-soluble, whatever your pet can't metabolize flushes harmlessly through the pet's system. The same is true for people, by the way. If a child gets into a bottle of Flea Treats and eats them, the ingredients are harmless to the child. My pet has a skin condition. Every summer it gets these bumps and raw spots.
Many pets are allergic to fleas.
It's worse in summer, not so bad in the winter. Cats get scabby spots along their necks and down their backs. Again, the allergy is usually worse in the summer months. This seasonality may not present itself if you live in a very mild-winter climate. And if the fleas aren't on your pet, they can't bite to aggravate the allergy.