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excess amounts may cause weakness, nausea, bone pain, headache, vomiting, loss of appetite, weight loss, elevation of cholesterol levels, itchiness, reduced libido, excess urination and increased thirst. Additional side effects are reported by Medline Plus.com, such as dry mouth, ringing in the ears, vertigo, dizziness and calcium deposits found in the organs in susceptible individuals.
Read more: http://www.livestrong.com/article/67766-side-effects-high-doses-vitamin/#ixzz2F2NDLQb5

Too much vitamin D can cause nausea, vomiting, poor appetite, constipation, weakness and weight loss. It can also raise blood calcium levels, causing changes in mental status such as confusion. High blood calcium can also cause abnormal heart rhythm. Too much vitamin D over a long time can cause depression, headache, sleepiness, and weakness, as well as calcium and bone loss. It can also cause the arteries and other soft tissues of the body (such as kidneys, heart, lungs) to become hardened and lined with layers of calcium. This condition is known as calcinosis.