Yes, cats can eat human-grade B-complex vitamins; the ingredients are the same. However, it's difficult to control the dosage for human medications, as humans are much larger and weigh dozens of times more than cats. Each tablet needs to be divided into very small portions for feeding, and the vitamin content is less specific. So, while it's possible, it's not recommended. The best option is to use cat-grade vitamin tablets. 
The reason is simple: cats cannot produce vitamins themselves, and as obligate carnivores, they typically obtain relatively few vitamins from their food. If you feed them vegetables and fruits, since they don't eat those, they usually lose interest after a couple of days. Vitamins are divided into four main types (A, B, D, and E). Vitamin deficiencies in cats can range from mild to severe. Vitamin A deficiency most commonly causes dry eye, resulting in excessive eye discharge and rough skin.
When should cats take vitamin B?
Generally, when a cat has a skin condition, it needs extra vitamin B supplementation. There's no need to give them a large dose of vitamin B at other times. If you wish to supplement their vitamin intake, under healthy conditions, a daily multivitamin tablet containing B vitamins is sufficient.
