Vitamin B Complex
A B complex bundles the whole B group in one tablet, which is tidy if you would rather not juggle separate B6, B12 and folic acid. They are popular with people chasing an energy lift, though it is worth being clear: B vitamins help your body release energy from food, they are not a stimulant. If you are genuinely low, the difference is real; if you are not, do not expect fireworks.
Common questions
Will a B complex give me energy?
Only if you were short to begin with. B vitamins are part of how the body turns food into energy, so a deficiency can leave you flat, but topping up when you are already fine will not act like caffeine.
Complex or single B vitamins?
A complex is convenient and covers the group. If you have been told you are specifically low in one, say B12, a targeted single dose makes more sense than the whole set.