- It won't make any effect for around month once you start taking it
- it only has effect if you have serious deficiency.
I used to be sick really easily for years, always tired. Finally got doctor to make blood tests and if normal level 100, min is 60, then my vitamin D levels were 12. Ofcourse my body immune system struggled. After started with suppliments got more energy, got less often sick. Felt like a normal person.
There is also other extreme, taking too many will have negative effect. I've experienced them myself.
I do live in a northern climate (imagine Finland) with long dark winters and only sun time is spend in the office.