There are plenty of studies that show multivitamins are a waste of money.
The important thing to do is to find out what, if anything, you are deficient in. And that means a lab test.
Vitamins are medicine, and have can have detrimental consequences: as just one example, let's say you think you are low in iron, even though you haven't had a lab test done to confirm it, so you decide to take an iron supplement - because iron dosages interfere with zinc absorption, you can create a zinc deficiency.
First check your B12, zinc, iron, and other vitamins and minerals by lab test and then decide how to supplement.
I'm a big believer in evidence-based medicine. Supplements need to be approached carefully. Don't take anything unless you know you need it because if you don't, it can hurt you. Of course, vice versa is true. If you are low in something, then you need to know and after supplementing that something for a month, you need to do another lab test to make sure you are supplementing enough (and not too much).
My daughter had a gazillion lab tests done the past few years to check the effects of meds, vitamins, and mineral supplements on her system - we saw her zinc depeleted by iron supplements but if she took too much zinc, then her iron went down.
Told she was deficient in Vitamin D, she with the blessing of her doctor tried different dosages - beware, because too high a dose for her resulted in hypercalcemia and other issues...now she takes 600IU daily of vit D, no more and no less - I agree with the US government study done last year which found that the way labs that routinely diagnose vitamin D deficiency has no science behind it. If your PTH and D2 in your body are normal, then it is unlikely you are deficient in vit D, no matter what the lab said...specialists in the US like endocrinologists tend to know this but the regular GI or GP is clueless. And the science in that study (which was actually an analysis of hundreds of studies done so far on vit D) suggests that 600IU daily is more effective than 1000IU or higher.