What are the side effects of taking vitamin d?

Usually none: Vitamin d is not associated routinely with side effects. If you take very high doses of vitamin d there is a very risk of high blood calcium levels (which can cause dehydration, thirst, and confusion) but this only occurs rarely and at doses >100 times the recommend daily intake. There is an activated, prescription form of vitamin d called Calcitriol that can cause high calcium at lower doses.

Relatively few: Vitamin d is an essential material for our body. If you are vitamin d deficient, there should be no side effect to taking vitamin d, although I have heard of patients getting constipated from the calcium supplementation that goes along with it. If you take very high levels of vitamin d (>20, 000 units per day), you can develop hyper calcemia which has a number of consequences. Consult a doctor.