4000 ius or more with good dietary sources and lots of sunshine in a younger person might be trouble. Occasionally I forget to take it too! It's winter here now and I'm older and carry a bit of extra weight, all require extra vitamin D. I'm doing other things too to improve my health like exercise, eating better and so help me Jesus a diet! The vitamin D applies to older people in northern climates, especially those with dark skin, large numbers of older people are deficient in vitamin D, better to make sure you have enough, I figure this is a safe winter range for me. The benefits of taking it are substantial for many and the reduction in infection rates and severity of illness make it worth taking, I don't have a general belief in supplements. this is supported by good evidence and expert opinion.. BTW Older folks make much less vitamin D from sunshine than younger ones, make sure your parents etc are taking it or know about it, many don't.