> But unfortunately if you are taking a vitamin because you lack a nutrient
Fortunately most people are not lacking a nutrient. Also fortunately most (but not all - the exceptions cause problems when they become the hype!) vitamins people take in large quantities are water soluble and you just pee out extra.
I tend to agree. But there are some like prenatal vitamins which people should be taking because dipping low on the nutrients can cause lifetime damage to the kid.
In that case, I worry less that the vitamin contains too much and more that it doesn't contain enough. Both could be a problem. There are also a few like iron which take longer for the body to purge where overdosing is a real problem.