The secret to staying young is broccoli and cabbage

I have a hard time believing vitamin supplements work that well and multivitamins in particular. You may end up thinking they are effective but you should have to work hard to get there. There are several major conceptual problems with vitamin supplements (this is before we get to multivitamins).

First is bioavailability. The gut isn't just a large database processing engine in which food is disassembled and its constituents simply extracted. Many compounds don't even survive the stomach acids. Different digestive stages extract different components which have to survive intact that far. In addition, some chemicals (vitamins) are synthesized in the body and don't pass from the gut at all (you mention B, which can be the opposite: B12 is synthesized in animal tissue but is not available to the body in that form; eating animals allows the gut to extract it and supply it to cells for use).

In addition, typically 95+% of what you eat or take (yep, those medicines too) is immediately removed by the first pass metabolism, so most of the drug you take are pissed into the toilet. Unless the compounds you eat are structurally appropriate to make it through the gut properly you'll lose them.

But does it matter? The second problem is dosage. You can take a worthlessly low amount or a dangerously high amount of many chemicals: how do you know the ultimate levels of bioavailable vitamin are useful? Studies in this area are largely, at best, poor.

Which leads to the multivitamin itself -- they always promise several X the "recommended daily consumption" but you have no idea what they do in combination, where those recommendations came from (mostly pulled out of someone's backside) and how much ends up usefully in your bloodstream. Not to mention most are not produced under GMP and congress has explicitly removed the FDA's oversight -- so you don't even know if you're getting what it says on the bottle.

Now I do selectively take two supplements: D1 and EPA+DHA. I take them both in an oil suspension (they are oils) and based on some research in German I've read and discussed with my doctors and some specific family history (half of which grew up at a much lower latitude, so I don't synthesize enough D in California). But finding a dose that gave the response we were looking for in my bloodstream was quite difficult and I take about 4X the US recommendation. Even then the data on proper resulting D levels is quite poor.

I think it is usually better to get one's nutrients from food, but Niacinamide is one supplement that I take regularly. A friend finds that it reduces her need for tobacco.

There are lots of nutrients in the foods that aren't in supplements.