Yes, I understand you can OD. Cannot recall where I read that 5000iu was a safe and effective dose, cos it was years ago. Avocado knows a lot about the subject.
Yes you can OD on vitamin D, though apparently it takes an awful lot and over a period of several months......however, when I first started take D I was taking 4,000 IU per day over the winter months. My D test came back at 384 nmol/L which is OD (upper lab reference range usually 150nmol/L or 200nmol/L depending on the lab). MY GP freaked. No harm was done though and all I needed to do was to stop taking it for two months and stay out of the sun. I now take 2,500 per day in winter and that dose every other day in summer. I am prescribed vitamin D by GP but have decided to buy my own because that way I get a steady dose each day rather than one big dose once a week (apparently it is more beneficial for the heart to take a dose each day). The only way to be sure of the correct amount of D for you is to get tested and pay for the test if necessary as some do - you can send away for one.
This video by Dr Michael Holick is well worth the hour long watch: https://www.youtube.com/watch?v=2lRtx-4_FOY
Also talks about OD and how rare it is. Dr Holick is American so the level of D is measured in ng/ml, something like that, conversion: divide nmol/L by 2.5 to change to ng/ml, and multiply by 2.5 to change ng/ml to nmol/L.
It can take up to five or six weeks to get an appointment at our GP surgery. Atrocious. Patients can see a nurse practitioner for five minutes in a sit and wait clinic but even that gets booked up. Patients are advised to go to a walk-in centre or A&E - so there, you know why A&Es are so busy as GPs are sending patients to them