One of the downsides of a market-driven natural products industry is the lack of regulation and oversight; I know of many examples of poor quality control in natural products, ranging from mislabeled quantities all the way to capsules of pathogenic bacteria and products contaminated with lead and mercury. Thus, I tend to be conservative regarding manufacturers. I only use and recommend products that have been tested and passed the rigorous quality control standards of the institution I trained at, Bastytr University, or other places I worked. There they require independent assays of ingredients as well as certifications of good manufacturing process and quality control.
This is moreso the case with herbal products than vitamin/minerals. Most vitamin preperations are simple and a well-known brand name should be fine (like emergen-C). There are obviously many more brands out there that have high standards; I just don’t have the time to do the lengthy detective work that it requires.Some brands I recommend are Enzymatic Therapy/Phytopharmica, Metagenics, NF Formulas, Thorne, Tyler, Eclectic Institute, Ayush herbs, Vital Nutrients, Carlson’s (fish oil), Now, Wise Woman Herbs, Nordic Naturals…(there are more).
Online, you can check out www.consumerlabs.com for tests of widely available dietary supplements. They tend to test the ‘mainstream’ brands, so they may be full of fillers and other non-necessary ingredients.