- 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.

So I am skeptical of supplements as a rule. Of course there are a couple of odds, like iodine, which is so beneficial to supplement in general community, that it is added to table salt as default, you only skip if you have specific conditions like thyroid problems.

I wish there were more research or comments on possible harms and disadvantages of excessive use of vitamin D, than I think I could more easily decide to use.

"however, findings are inconsistent"

If you follow the daily dosage information, you won't even get near harmful levels of vitamin d. Recommended is about 600 IU daily. If you take 60.000 on a daily basis for months, you build up toxicity.

For that you typically need to take 30 pretty strong 2000 UI pills on a daily basis for months, while ignoring the label that states each pill is typically 250% of the daily recommended amount, and the instruction which notes the tolerable levels max out at just two daily pills.

Toxicity is extremely uncommon. Whereas deficiency is extremely common. Up to a billion people are expected to have some form of vitamin d deficiency.

That's not to say it's not a problem, but rather that it's very rare, nothing like say an opioid overdose problem. As with all things, balance is key. You can overdose on 5 cents of water in the span of 5 minutes. Acute vitamin d overdose is known to happen if you take something like 300-1000 pills of 2000 UI over a period of days, which is pretty insane.

https://en.wikipedia.org/wiki/Hypervitaminosis_D

The NHS does recognise vitamin D deficiency as an issue.

In the UK, a science advisory body (Scientific Advisory Committee on Nutrition) published a 300 page report of the evidence on vitamin D and health in 2016. Their conclusion: adults and children over one in the UK should have at least 10 micrograms of vitamin D every day. For some people this may mean taking a vitamin D supplement. People who have a higher risk of vitamin D deficiency are advised to take a supplement all year round.

Here is an overview of the guidelines on the NHS website which includes a link to the 300 page scientific review.

https://www.nhs.uk/news/food-and-diet/the-new-guidelines-on-...