I take NatureMade brand vitamin-D supplements. That brand is USP certified which means their product is externally certified (through products purchased at sto

I take NatureMade brand vitamin-D supplements. 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-supplements-wisely/ https://www.consumerreports.org/supplements/how-to-choose-su...