That brand is USP certified which means their product is externally certified (through products purchased at stores) to contain the amount of supplement they say it has.
There are some other similar certification programs that you can look for when choosing supplements, but USP seems to be the most rigorous:
https://www.consumerreports.org/supplements/how-to-choose-su...
I've been using a D3 supplement of 5000 IU from NatureWise, for several years. I take one per day depending on the time of year. It's inexpensive, ~$11-12 for a year supply, and it has worked very well. I take it daily throughout Fall & Winter, and then lighten up on it during Summer.
https://originmaine.com/nutrition/jocko-d3-gel-capsules-360c...
I was taking 1,000mg a day and that didn't seem to move the needle on my Vitamin D blood work.
In recent weeks I've been taking 4,000mg/day (with K2), which is 4x what I've had pharmacists recommend.
I plan to keep that up for a couple more weeks while I'm under quarantine (recently flew from a high-risk country to a less high-risk country). After that I'll level off to 2,000mg a day.
But I'm shooting in the dark. I have no idea if 2,000mg/day or 4,000mg/day is too much or not enough.
- Mayo Clinic says "Taking 60,000 international units (IU) a day of vitamin D for several months has been shown to cause toxicity."
- news-medical.net says "To prevent toxicity, tolerable upper intake levels have been fixed for vitamin D as..." 4000 for adults.
- healthline.com says "A daily vitamin D intake of 1000–4000 IU (25–100 micrograms) should be enough to ensure optimal blood levels for most people."
https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...
https://www.news-medical.net/health/Vitamin-D-Overdose.aspx
https://www.healthline.com/nutrition/how-much-vitamin-d-is-t...
https://www.healthline.com/nutrition/vitamin-d-from-sun#time...
Amazon has a few options in the $50-90 range.
You prick your finger, draw a "small amount" of blood, and send it to a lab.
It's very cheap and safe, so worst case it has less effect than you hope. You can overdose, but you have to really crazy with the dosage.
Also 1: Vitamin D supplements do cure rickets.
Also 2: All of us who eat supplements should show up in the stats as high in vitamin D, so the data does partially cover that case.