Vitamin D is a fat soluble vitamin that is stored in the body. Unlike other fat soluble vitamins it is pretty difficult to overdose on vitamin D. Since it is not a drug but a vitamin and a very important one at that, it does not have the usual side effects you associate with "drugs". When we spend enough time outdoors, our bodies naturally produce about 10,000 IU with no problems, especially for people who spend a lot of time in the sun or who live close to the equator.
What are some side effects of vitamin D3? Well none unless you constantly take too much or have some other pre-existing health conditions or an allergy to vitamin D3. At the lower end of the side effect spectrum you have constipation, vomiting, general weakness, nausea, mild dizziness, lethargy, frequent urination, extreme thirst, nervousness, itching, anorexia, heart arrhythmias, polyuria (excessive urinating), and weight loss and are usually associated with poor quality supplements. These can usually be reversed by taking a better quality supplement and or lowering the amount of vitamin D3 you take.
Natural Vitamin D
For those that chronically take an overdose of vitamin D3 over a long period of time can cause themselves some serious discomfort. The problem is that the body can't use up all the calcium, especially without other trace nutrients helping to use it. Unabsorbed calcium running through the body can cause bladder and kidney stones. It also can restrict fluid circulation in the body causing kidney failure, hypercalcaemia (high levels of calcium) and even anorexia.
Check with your doctor before you take a vitamin D3 supplement if you have any pre-existing conditions. People with pre-existing conditions such as hypercalcaemia and difficulty absorbing calcium should not take excess amounts of a calcium supplement; especially cheap supplements that do not get used but run through the blood. This can cause high blood pressure which can lead to heart disease. The interesting thing is, a high quality vitamin D3 supplement can do a lot to improve heart related problems and heal your body of other maladies.
Some supplements are not made with absorption in mind and come out looking as they did going in. These supplements can cause problems for people with diseases of the kidney, liver, heart and cause immune system disorders such as sarcoidosis. Also, check with your doctor if you have intestinal or digestive complications like Whipple's or Crohn's disease, low levels of bile, or little or no stomach acid before taking a vitamin D supplement.
The only thing not associated with overdose that could cause side-effects with vitamin D3 concern allergic reactions. Symptoms include severe dizziness, difficulty breathing, rashes, and swelling of the face, tongue, or throat. These require immediate medical attention.
More and more studies revealing the benefits of vitamin D3 are being done all the time. It is wise to look into them and see if taking a high quality vitamin D3 supplement is right for you.