Which has zero bearing on the parent's point: supplements (which includes vitamins) aren't regulated making them less safe. There's an incentive to adulterate

Which has zero bearing on the parent's point: supplements (which includes vitamins) aren't regulated making them less safe. There's an incentive to adulterate these products with anything producing noticable positive short-term effects. There's no reason to believe vitamins are exceptional here - if you start taking a multivitamin that seems to improve your focus and energy slightly, you're more likely to continue buying that vitamin. A small addition of Amphetamine salts or even caffeine would do the trick.