Last winter, I wound up sick due to low vitamin D. We are talking "had to sit while making a simple dinner" sort of sick, headaches and backaches and so on. Now, stronger vitamin D pills are prescription here, lower-dose aren't. I got prescription pills to take for 90 days. I had a blood test after 1.5 to 2 months (earlier than initially thought, but the doctor was curious.

Not only were my vitamin D levels good, but I felt much better. Better than I had in months. I still take the lower dose pills to prevent such a thing. I have the choice to get sun during the summer - at least 15 minutes a day with sun on more than just my face. Alas, I find the temperatures too cold to do that all the time (I live in Norway, summers are chilly). On the other hand, my levels are still good (had them tested last month per doctor's orders).

Now, it might take a bit longer to actually repair damage done by low vitamin D, but as far as how I felt - it was much quicker.