The tricky thing about “medicines” like vitamin D is that you don’t immediately overdose on them. If you take too much Tylenol, you end up in the hospital pretty fast. That giant vitamin capsule isn’t likely to land you in the ER tomorrow or even a month from now. But vitamin D, unlike many of the other vitamins you may be taking, is fat soluble. That means that if you take too much of it, you won’t just pee it out like you would a water soluble vitamin. Instead of being carried out in your body's wastewater, the vitamin will cling to your body fat for later use—which can compound the effects of daily overdosing. But it's easy to think of supplements as harmless when you're just popping a pill or two in your bathroom each morning.