Morning is the best time to take a multivitamin, but if you take it with your coffee, you won't derive its full benefits because caffeine has a diuretic effect. Since most of the nutrients in a multivitamin are water-soluble, much of your supplement will be flushed down the toilet (quite literally!). If this is the case, wait a couple of hours and take your multivitamin with your mid-morning snack or lunch.