I buy the NOW brand (not a promo - just it works for me) online. I used to use Jarrow, but they quit making it a few years ago, so had to look for another reputable source.
Bought another brand and it didn't seem to work the same - later found out they were a company "repackaging and rebranding" bulk supplement from overseas, and upon testing some of their stuff wasn't as advertised. This is just one example of "buyer beware" when it comes to supplements - don't go with the lowest price or an unknown brand. I look for a US company with independently inspected facilities. There are several others beside NOW.
I use 600 mg twice a day.
Sue