My vitamin d level is 18. why vitamin d supplement gives me horrible muscle cramps. Should I take calcium with vitamin d or take vitamin d only?

Unusual response. This is a very unusual reaction to Vit D. You are more likely to be reacting to the other ingredients in the supplement so look for a pure Vit D3 with minimal additives. Most adults need 5-8000 iu/day to get optimal levels of 50-80. You probably don't need calcium unless your diet is low in calcium but it is wise to take Vit K with Vit D. Muscle cramps may be caused by low Magnesium. See comment: