It is true that many people, especially tznius women with little sun exposure, are Vitamin D deficient, and that taking vitamin D supplements can help may issues. My Dr advised me to take supplements when I complained about being tired and getting headaches. But you can overdose on vitamin D, since it is not water soluble it can build up in your body and is not excreted.
some quick googling about vitamin D:
from the mayo clinic: “The main consequence of vitamin D toxicity is a buildup of calcium in your blood (hypercalcemia), which can cause poor appetite, nausea and vomiting. Weakness, frequent urination and kidney problems also may occur….taking 50,000 international units (IU) a day of vitamin D for several months has been shown to cause toxicity. This level is many times higher than the Recommended Dietary Allowance (RDA) for most adults of 600 IU of vitamin D a day.”
WebMD says that current recommendations are that up to 4,000 IU/day are safe, but some studies suggest that up to 10,000 per day are still safe for a healthy adult. But there is also no proof that megadoses provide any health benefit.
Sparkly, as far as I know, vitamin D does not cause cancer- in fact some studies say that it protects against some kinds of cancer. It is also necessary for keratinocyte development. Exposure to sunlight (UV) can do 2 things- it is a natural source that allows the body to produce vitamin D, which is good, but UV radiation can also damage DNA in skin cells (e.g. melanocytes) thereby causing cancer (melanoma), which is why people prefer to get their needed vitamin D from supplements rather than sun exposure. The vitamin D does not cause the cancer, the UV does. melanin actually absorbs the UV rays; which is why darker people need more UV exposure to make enough vitamin D than fair people, but on the flip side, fairer people are more prone to skin cancer.