I've taken as high as 3000IU, and have heard that as much as 5000IU can be taken. Then going down from there, after say a month. This is to load the body with vitamin D3, a loading dose. D3 in a capsule form is more easily assimulated by the body, just D isn't as well utilized. Sunlight on the skin is converted into D3, so if you spend a lot of time outdoors, you may not need as much, as say an office worker. If you live in the North, I would venture to say that you are deficent. If, while taking the higher dose, you notice easy bruising, it may be that your blood is too thin. High doses of D3 should not be combined with coumadin (warfarin), without consulting a doctor. It is best to take D3, with calcium. Some say it can cause toxicity, some say it does not. The signs of toxicity include the following:
Calcification of soft tissue, frequent thirst, weakness, and loss of appetite.
Signs of deficiency include rickets, rhemumatic pains, burning mouth syndrome, insomnia, visual problems, and hypothyroidism. The dose most recommended is a 1000 IU.
A maintenance dose as most refer to it. I don't feel the RDA of 400IU is high enough, unless you live in a warm climate, such as is the case in the South.