I used to take a kids' multivitamin, but that was a while ago. I don't take one now. I think ideally people should get their vitamins from what they eat, but a multivitamin can't hurt. I heard that some of the vitamins really just pass through you, though.
The data I have read on that is mixed. However if you are going to take them studies show you should take them with your biggest meal of the day. Some vitamins and/or minerals need others nutrients to be absorbed by the body. You stand a better chance of matching those nutrients by taking the vitamins with a larger more diverse meal.
Most of the time it just helps you make really expensive urine. Unless you're actually medically malnourished.
Occasionally.
I keep a B vitamins pill around. I take them occasionally.They are of course water soluble. When my urine turns bright yellow, I stop taking them. Since that sometimes takes a few days, I assume that I did need them at the time.
Is how I use them necessary or helpful?---not really sure, but I feel like I am doing something positive for myself and that's usually a good thing regardless.
If you have questions about you personally taking any over the counter items I suggest you discuss it all with your doctor.
It's really opinionated. Some people say YES! And some people are ABSOLUTELY NOT! I say, if you're healthy, you exercise and you sleep, eat well and are living a good, stable, healthy life, then multivitamins are probably not necessary. But some people, despite having a stable, healthy life enjoy taking multivitamins because it makes them feel better. So it's a real personal preferance.