i take one everyday... i seem to have a problem with not eating enough in a day, so i take it just cuz my body needs it... i stick with flinstone's chewables, no matter what anyone says.. an adult can still take them
I assume that you're asking about the pill you by at the store and aren't simply wondering if your body really needs those chemicals.
Vitamins will make sure that body doesn't have any major deficiencies - that's their only job and they 'work' in that respect. If you're asking if zinc improves your sex life or Vitamin C helps you fight a cold that would depend on how much you were taking and how it was reacting in your body.
Also, vitamins don't help you gain weight, there are certain supplements that do that though. Is that what you were talking about?