My husband was feeling like his muscles were weak and tired so I suggested he had a vit d blood test. His levels came back at 39 ng/ml. This is not in the deficient range but is insufficient. The NHS recommend levels above 50. After 1 week of taking 2500iu per day he was feeling much better. After 3 months he did another test and his levels came back at 67ng/ml...perfect! It just goes to show that you can have symptoms of lack of vitamin d before you actually get into the deficient range. He never went out in the sun as he was working all the time so it shows that these tablets work!