It appears that about 1 person in 300 will have an allergic reaction vitamin D
   (Have no specific reference for that ratio - it is just an impression from reading 3,000 articles on vitamin D)
The reaction may include itchy skin, hives, intestinal upset.
Reactions occur within 3 days of taking vitamin D - sometimes within a day
The reaction is real but for unknown reasons, and there are many possible solutions
Hope by the end of 2012 to have narrowed the solutions down to just a few.

Possible reasons and some potential solutions

• Reaction is due to lack of Magnesium - which is brought out by vitamin D
  Take Magnesium supplement
• Similar to reaction to being in the sun
• Perhaps need to start with a smaller dose
  Some doctors start the patient on 50,000 IU of vitamin D approximately weekly
  Might try having daily instead of weekly doses
• Might be a reaction to Vitamin D2 - which is still the only form allowed to be prescribed in the US
• Might be a reaction by the gut to Vitamin D
  Try a sublingual form or one which is easier to digest (see below)
• Reaction to a prescription drug
• Some speculation that the reaction is the the oil or the gelatin capsule

Solutions which might apply to many of the above

• Make sure that you are taking D3 and not D2
• Try a smaller amount
• Try a different form of vitamin D: bioemulsified
• Try sun or UV instead of vitamin D (but not if have an allergic reaction to the sun)

Vitamin D Life recommended way to try vitamin D (virtually no doctor is so cautious)

1. Take a small amount of vitamin D ( say 1,000 IU or a portion of a capsule)
2. Wait for 3 days to see it there is any allergic reaction
3. If no reaction, proceed with the maintenance dose (2,000 - 10,000 IU daily) or loading dose (50,000 IU 1 to 10 times per month)

See also Vitamin D Life

• All items in category Interactions 39 items as of Dec 2011 (virtually all interactions = Drug reduces the vitamin D effectivity, no reaction
• Vitamin D interactions with drugs – Oct 2011 Abstract says which drugs, but not amount of interaction
• NLM Vitamin D recommendations and interactions – Jan 2011
• Side Effects and Warning for vitamin D
  Excessive thirst, Metal taste in mouth, Poor appetite, Weight loss, Bone pain, Tiredness, Sore eyes, Itchy skin,
  Vomiting, Diarrhea, Constipation, A frequent need to urinate, Muscle problems
• On-line survey of drug uses and side effects – including vitamin D

Which contains the following chart:

Note the following are not the % of people taking vitamin D who got reactions

but rather the % who had a any reaction which had a particular reaction

Reaction to Aspirin is about 3X more likely than allergic reaction to vitamin D

• How Do I Know if I Am Allergic To Aspirin? 1%
• Aspirin - Not just for pain relief anymore 1%
• An Aspirin Allergy and its Relationship to a Salicylate Allergy 1%?